feat: global Docker health indicator and graceful degradation

- GET /api/health endpoint returning Docker connectivity status
- Sidebar shows Docker connection dot (green=connected, red=disconnected)
- Stale scanner returns store-only results when Docker is unavailable
- Polls health every 30s

internal/api/router.go

@@ -130,6 +130,7 @@ func (s *Server) Router() chi.Router {
 			r.Use(auth.Middleware(s.localAuth))
 
 			// Read-only endpoints (any authenticated user).
+			r.Get("/health", s.getHealth)
 			r.Get("/auth/me", s.currentUser)
 			r.Get("/projects", s.listProjects)
 			r.Route("/projects/{id}", func(r chi.Router) {