refactor(stream): route audio/realtime effect-processing error through tracing

The realtime effects loop in src/audio/realtime.rs was using
`eprintln!` to surface effect processing errors. That bypasses the
tracing subscriber and so the error never reaches the OTel collector
or the structured-log pipeline — invisible to operators in prod.

Switched to `tracing::error!` with the error captured as a structured
field, matching the rest of the stream server.

Why this was the only console-style call to fix:
The earlier audit reported 23 `console.log` instances across the
codebase, but most were in JSDoc/Markdown blocks or commented-out
lines. The actual production-code count, after stripping comments,
was zero on the frontend, zero in the backend API server (the
`fmt.Print*` calls live in CLI tools under cmd/ and are legitimate),
and one in the stream server (this fix). The rest of the Rust
println! calls are in load-test binaries and #[cfg(test)] blocks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

veza-stream-server/src/audio/realtime.rs

@@ -8,6 +8,7 @@ use parking_lot::{Mutex, RwLock};
 use std::collections::VecDeque;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
+use tracing::error;
 
 /// Buffer circulaire thread-safe pour audio temps réel
 #[derive(Debug)]
@@ -319,7 +320,7 @@ impl RealtimeAudioProcessor {
                     config.sample_rate,
                     config.channels,
                 ) {
-                    eprintln!("Effect processing error: {:?}", e);
+                    error!(error = ?e, "Effect processing error");
                 }
             }