refactor(backend): J3 — remove 3 deprecated unused handlers

Cleanup of dead code marked // DEPRECATED in veza-backend-api/internal/handlers.
Each symbol was verified to have zero callers across the codebase before
deletion (go build ./... + go vet ./... + go test ./internal/... pass).

Deleted:
- UploadResponse type (upload.go) — callers use upload.StandardUploadResponse
- BindJSON method on CommonHandler (common.go) — callers use BindAndValidateJSON
- sendMessage method on *Client (playback_websocket_handler.go) —
  internal WS broadcast now goes through sendStandardizedMessage

Kept as tech debt (still actively used, refactor out of J3 scope):
- UploadRequest type (upload.go:23) — used by upload handler, refactor
  requires migrating to upload.StandardUploadRequest with multipart binding
- BroadcastMessage type (playback_websocket_handler.go:53) — still the
  channel type for legacy playback broadcasts and referenced in tests

Also in this day (already committed in parallel):
- veza-backend-api/internal/api/handlers/two_factor_handlers.go deletion
  (had //go:build ignore, zero callers) — bundled into 7fa314866 by
  concurrent work on .github/workflows/*.yml

seed-v2 investigation:
- No Go source for seed-v2 found — it was only a compiled binary
  already purged in J1 (0e7097ed1). No code action needed.

Refs: AUDIT_REPORT.md §8.1, §12 item 1-2

veza-backend-api/internal/handlers/common.go

@@ -195,19 +195,6 @@ func BuildPaginationDataWithCursor(limit int, total int64, nextCursor, prevCurso
 	}
 }
 
-// BindJSON lie les données JSON de la requête à une structure
-// DEPRECATED: Utiliser BindAndValidateJSON à la place pour une gestion d'erreurs robuste
-func (h *CommonHandler) BindJSON(c *gin.Context, obj interface{}) error {
-	if err := c.ShouldBindJSON(obj); err != nil {
-		h.logger.Warn("Failed to bind JSON",
-			zap.Error(err),
-			zap.String("request_id", c.GetString("request_id")),
-		)
-		return err
-	}
-	return nil
-}
-
 // BindAndValidateJSON lie et valide les données JSON de la requête de manière robuste
 // P0: JSON Hardening - Garantit qu'aucune erreur de parsing/validation ne passe silencieusement
 //

veza-backend-api/internal/handlers/playback_websocket_handler.go

@@ -252,27 +252,6 @@ func (c *Client) writePump() {
 	}
 }
 
-// sendMessage envoie un message au client (legacy format)
-// DEPRECATED: Use sendStandardizedMessage instead
-func (c *Client) sendMessage(msg *BroadcastMessage) {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-
-	data, err := json.Marshal(msg)
-	if err != nil {
-		c.handler.logger.Error("Failed to marshal message",
-			zap.Error(err),
-			zap.String("user_id", c.userID.String()))
-		return
-	}
-
-	select {
-	case c.send <- data:
-	default:
-		close(c.send)
-	}
-}
-
 // sendStandardizedMessage envoie un message au client avec format standardisé
 // INT-014: Standardized WebSocket message format
 func (c *Client) sendStandardizedMessage(msg *wsmsg.WebSocketMessage) {

veza-backend-api/internal/handlers/upload.go

@@ -29,20 +29,6 @@ type UploadRequest struct {
 	Metadata string `form:"metadata"`
 }
 
-// UploadResponse réponse pour upload
-// DEPRECATED: Use upload.StandardUploadResponse instead
-// INT-015: Kept for backward compatibility during migration
-type UploadResponse struct {
-	ID        uuid.UUID `json:"id"`
-	TrackID   uuid.UUID `json:"track_id"`
-	FileName  string    `json:"file_name"`
-	FileSize  int64     `json:"file_size"`
-	FileType  string    `json:"file_type"`
-	Checksum  string    `json:"checksum"`
-	Status    string    `json:"status"`
-	CreatedAt time.Time `json:"created_at"`
-}
-
 // UploadValidatorInterface définit les méthodes nécessaires pour UploadValidator
 type UploadValidatorInterface interface {
 	ValidateFile(ctx context.Context, fileHeader *multipart.FileHeader, fileType string) (*services.ValidationResult, error)