feat: add observability improvements (metrics, audit log, structured logging)

- Add Prometheus metrics package (internal/metrics) with deployment,
  container health, webhook, HTTP request, and audit counters/histograms
- Add audit_log SQLite table via migration 007
- Add AuditEntry model with CRUD operations and query methods
- Add audit service (internal/service/audit) for recording user actions
- Instrument deploy service with deployment duration, count, and
  in-flight metrics; container health gauge updates on deploy completion
- Instrument webhook service with event counters by app/type/matched
- Instrument HTTP middleware with request count, duration, and response
  size metrics; also log response bytes in structured request logs
- Add audit logging to all key handler operations: login/logout, app
  CRUD, deploy, cancel, rollback, restart/stop/start, webhook receipt,
  and initial setup
- Add GET /api/audit endpoint for querying recent audit entries
- Make /metrics endpoint always available (optionally auth-protected)
- Add comprehensive tests for metrics, audit model, and audit service
- Update existing test infrastructure with metrics and audit dependencies
- Update README with Observability section documenting all metrics,
  audit log, and structured logging

internal/server/routes.go

@@ -115,14 +115,13 @@ func (s *Server) SetupRoutes() {
 			r.Get("/apps", s.handlers.HandleAPIListApps())
 			r.Get("/apps/{id}", s.handlers.HandleAPIGetApp())
 			r.Get("/apps/{id}/deployments", s.handlers.HandleAPIListDeployments())
+			r.Get("/audit", s.handlers.HandleAPIAuditLog())
 		})
 	})
 
-	// Metrics endpoint (optional, with basic auth)
-	if s.params.Config.MetricsUsername != "" {
-		s.router.Group(func(r chi.Router) {
-			r.Use(s.mw.MetricsAuth())
-			r.Get("/metrics", promhttp.Handler().ServeHTTP)
-		})
-	}
+	// Metrics endpoint (always available, optionally protected with basic auth)
+	s.router.Group(func(r chi.Router) {
+		r.Use(s.mw.MetricsAuth())
+		r.Get("/metrics", promhttp.Handler().ServeHTTP)
+	})
 }