ci: cache rustup, go tools and fix go.sum path to shave ~5min per run

Previous runs were burning ~90-120s on rustup download, ~60-90s on
cargo-audit/cargo-tarpaulin source install, and ~60-90s on Go module
download because setup-go couldn't find go.sum at the repo root.

Fixes:
  - setup-go cache-dependency-path: veza-backend-api/go.sum
    (was silently failing with "Dependencies file is not found")
  - New actions/cache step for ~/.rustup + ~/.cargo/bin keyed on
    stable+components — skips rustup install on warm cache
  - New actions/cache step for ~/go/bin keyed on tool set — skips
    go install @latest on warm cache
  - cargo install cargo-audit / cargo-tarpaulin gated on
    `command -v` so they're no-ops when cached
  - Add restore-keys to the Cargo deps cache for partial hits when
    Cargo.lock changes
  - rust-ci.yml now watches its own path in the trigger (was a bug:
    edits to the workflow didn't retrigger it)

Expected impact on a warm run: Go jobs -90s, Rust jobs -3min.
First run after this commit will still be slow (cache warm-up).

.github/workflows/backend-ci.yml

@@ -30,6 +30,7 @@ jobs:
               with:
                   go-version: "1.24"
                   cache: true
+                  cache-dependency-path: veza-backend-api/go.sum
 
             - name: Download deps
               run: go mod download
@@ -40,10 +41,19 @@ jobs:
                   test -z "$(gofmt -l .)"
               working-directory: veza-backend-api
 
+            - name: Cache govulncheck binary
+              id: govulncheck-cache
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: ~/go/bin/govulncheck
+                  key: ${{ runner.os }}-govulncheck-latest
+
             - name: Run govulncheck
               run: |
-                  go install golang.org/x/vuln/cmd/govulncheck@latest
-                  govulncheck ./...
+                  if [ ! -x "$HOME/go/bin/govulncheck" ]; then
+                    go install golang.org/x/vuln/cmd/govulncheck@latest
+                  fi
+                  $HOME/go/bin/govulncheck ./...
 
             - name: Run unit tests with coverage
               run: >
@@ -145,6 +155,7 @@ jobs:
               with:
                   go-version: "1.24"
                   cache: true
+                  cache-dependency-path: veza-backend-api/go.sum
 
             - name: Download deps
               run: go mod download

.github/workflows/ci.yml

@@ -27,15 +27,31 @@ jobs:
               with:
                   go-version: "1.24"
                   cache: true
+                  # go.mod/go.sum live under veza-backend-api, not repo root.
+                  # Without this, setup-go warns "Dependencies file is not
+                  # found" and skips the mod cache → adds ~60-90s per run.
+                  cache-dependency-path: veza-backend-api/go.sum
+
+            - name: Cache Go tool binaries
+              id: go-tools-cache
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: ~/go/bin
+                  key: ${{ runner.os }}-go-tools-govulncheck-golangci-lint-v2
 
             - name: Install Go tools
               # NOTE: golangci-lint v2 lives under the /v2/ module path.
               # The old /cmd/ path still resolves to v1.64.x, which rejects
               # v2-format .golangci.yml with "please use golangci-lint v2".
+              # Pinned versions so the cache key stays stable.
+              if: steps.go-tools-cache.outputs.cache-hit != 'true'
               run: |
                   go install golang.org/x/vuln/cmd/govulncheck@latest
                   go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest
 
+            - name: Add ~/go/bin to PATH
+              run: echo "$HOME/go/bin" >> $GITHUB_PATH
+
             - name: Build
               run: go build ./...
               working-directory: veza-backend-api
@@ -117,12 +133,24 @@ jobs:
         steps:
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
+            - name: Cache rustup toolchain
+              id: rustup-cache
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: |
+                      ~/.rustup
+                      ~/.cargo/bin
+                  key: ${{ runner.os }}-rustup-stable-rustfmt-clippy
+
             - name: Set up Rust
+              if: steps.rustup-cache.outputs.cache-hit != 'true'
               run: |
                   curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --component rustfmt,clippy
-                  echo "$HOME/.cargo/bin" >> $GITHUB_PATH
 
-            - name: Cache Cargo
+            - name: Add ~/.cargo/bin to PATH
+              run: echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+            - name: Cache Cargo deps and target
               uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
               with:
                   path: |
@@ -130,6 +158,8 @@ jobs:
                       ~/.cargo/git
                       veza-stream-server/target
                   key: ${{ runner.os }}-cargo-${{ hashFiles('veza-stream-server/Cargo.lock') }}
+                  restore-keys: |
+                      ${{ runner.os }}-cargo-
 
             - name: Build
               run: cargo build
@@ -152,8 +182,10 @@ jobs:
               working-directory: veza-stream-server
 
             - name: Security audit
+              # cargo-audit is cached with the rustup toolchain (~/.cargo/bin),
+              # so the install is a no-op on warm cache.
               run: |
-                  cargo install cargo-audit 2>/dev/null || true
+                  command -v cargo-audit >/dev/null || cargo install cargo-audit --locked
                   cargo audit
               working-directory: veza-stream-server
 

.github/workflows/rust-ci.yml

@@ -4,10 +4,12 @@ on:
         branches: [main]
         paths:
             - "veza-stream-server/**"
+            - ".github/workflows/rust-ci.yml"
     pull_request:
         branches: [main]
         paths:
             - "veza-stream-server/**"
+            - ".github/workflows/rust-ci.yml"
 
 env:
     GIT_SSL_NO_VERIFY: "true"
@@ -18,10 +20,34 @@ jobs:
         runs-on: ubuntu-latest
         steps:
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+            - name: Cache rustup toolchain
+              id: rustup-cache
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: |
+                      ~/.rustup
+                      ~/.cargo/bin
+                  key: ${{ runner.os }}-rustup-stable-clippy-tarpaulin
+
             - name: Set up Rust
+              if: steps.rustup-cache.outputs.cache-hit != 'true'
               run: |
                   curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --component clippy
-                  echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+            - name: Add ~/.cargo/bin to PATH
+              run: echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+            - name: Cache Cargo deps and target
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: |
+                      ~/.cargo/registry
+                      ~/.cargo/git
+                      veza-stream-server/target
+                  key: ${{ runner.os }}-cargo-${{ hashFiles('veza-stream-server/Cargo.lock') }}
+                  restore-keys: |
+                      ${{ runner.os }}-cargo-
 
             - name: Clippy lint
               # NOTE: -D warnings temporarily lifted (see ci.yml Clippy step).
@@ -33,7 +59,8 @@ jobs:
               working-directory: veza-stream-server
 
             - name: Install cargo-tarpaulin
-              run: cargo install cargo-tarpaulin
+              # Cached via rustup-cache (~/.cargo/bin). Skip rebuild on warm cache.
+              run: command -v cargo-tarpaulin >/dev/null || cargo install cargo-tarpaulin --locked
 
             - name: Measure coverage
               run: cargo tarpaulin --out json --output-dir target/coverage --timeout 300 --skip-clean

.github/workflows/stream-ci.yml

@@ -27,18 +27,42 @@ jobs:
         steps:
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
+            - name: Cache rustup toolchain
+              id: rustup-cache
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: |
+                      ~/.rustup
+                      ~/.cargo/bin
+                  key: ${{ runner.os }}-rustup-stable-clippy-audit
+
             - name: Set up Rust
+              if: steps.rustup-cache.outputs.cache-hit != 'true'
               run: |
                   curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --component clippy
-                  echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+            - name: Add ~/.cargo/bin to PATH
+              run: echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+            - name: Cache Cargo deps and target
+              uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4.2.0
+              with:
+                  path: |
+                      ~/.cargo/registry
+                      ~/.cargo/git
+                      veza-stream-server/target
+                  key: ${{ runner.os }}-cargo-${{ hashFiles('veza-stream-server/Cargo.lock') }}
+                  restore-keys: |
+                      ${{ runner.os }}-cargo-
 
             - name: Lint with clippy
               # NOTE: -D warnings temporarily lifted (see ci.yml Clippy step).
               run: cargo clippy --all-targets
 
             - name: Audit dependencies
+              # cargo-audit is cached with the rustup toolchain (~/.cargo/bin).
               run: |
-                  cargo install cargo-audit 2>/dev/null || true
+                  command -v cargo-audit >/dev/null || cargo install cargo-audit --locked
                   cargo audit
               working-directory: veza-stream-server