fix(audit): drain in-flight recording goroutines on shutdown (M-1)

Audit events spawned from the HTTP middleware ran in detached goroutines
using context.Background(). On SIGTERM the DB pool was closed before
those goroutines finished writing, silently dropping audit events
(CWE-662 Improper Synchronization / CWE-400 Uncontrolled Resource
Consumption).

NewAuditLog now returns an *AuditMiddleware struct that tracks every
spawned goroutine with sync.WaitGroup. Callers wire the middleware via
its Middleware method value (preserves the existing
func(http.Handler) http.Handler shape) and drain the WaitGroup with
Flush(ctx), which blocks until in-flight recordings complete or the
provided context is cancelled — mirroring scheduler.WaitForCompletion.

Flush is invoked in cmd/server/main.go between http.Server.Shutdown
(no new requests accepted) and db.Close (pool torn down), with a
timeout returning ErrAuditFlushTimeout wrapping ctx.Err().

Request-derived inputs (method, path, status) are snapshotted before
the goroutine spawn so the worker does not race with http.Server
reusing r after the handler returns.

Tests:
  TestAuditLog_FlushDrainsInFlightGoroutines
  TestAuditLog_FlushTimeoutReturnsErrAuditFlushTimeout

Verification:
  go build ./...                            : 0
  go vet ./...                              : 0
  go test -race -short ./...                : 0 (all packages)
  go test -cover ./internal/api/middleware  : 81.4%
  golangci-lint run                         : 0 issues
  govulncheck ./...                         : 0 vulns in called code

cmd/server/main.go

@@ -589,7 +589,7 @@ func main() {
 		bodyLimitMiddleware,
 		corsMiddleware,
 		authMiddleware,
-		auditMiddleware,
+		auditMiddleware.Middleware,
 	}
 
 	// Add rate limiter if enabled
@@ -606,7 +606,7 @@ func main() {
 			rateLimiter,
 			corsMiddleware,
 			authMiddleware,
-			auditMiddleware,
+			auditMiddleware.Middleware,
 		}
 		logger.Info("rate limiting enabled", "rps", cfg.RateLimit.RPS, "burst", cfg.RateLimit.BurstSize)
 	}
@@ -724,6 +724,17 @@ func main() {
 		logger.Error("HTTP server shutdown error", "error", err)
 	}
 
+	// Drain in-flight audit-recording goroutines before closing the DB pool.
+	// The audit middleware spawns one goroutine per non-excluded request; those
+	// goroutines run detached from the request context and write to the
+	// audit_events table via the same *sql.DB. Without this drain, SIGTERM
+	// would close the DB pool while recordings were mid-flight, silently
+	// dropping audit events (M-1, CWE-662 / CWE-400).
+	logger.Info("flushing audit middleware in-flight recordings")
+	if err := auditMiddleware.Flush(shutdownCtx); err != nil {
+		logger.Warn("audit middleware flush did not complete in time", "error", err)
+	}
+
 	// Close database connection
 	if err := db.Close(); err != nil {
 		logger.Error("error closing database connection", "error", err)