feat: complete phase 5 -- error hardening, frontend, Docker, demo, docs

Backend:
- ConversationTracker: Protocol + PostgresConversationTracker for lifecycle tracking
- Error handler: ErrorCategory enum, classify_error(), with_retry() exponential backoff
- Wire PostgresAnalyticsRecorder + ConversationTracker into ws_handler
- Rate limiting (10 msg/10s per thread), edge case hardening
- Health endpoint GET /api/health, version 0.5.0
- Demo seed data script + sample OpenAPI spec

Frontend (all new):
- React Router with NavBar (Chat / Replay / Dashboard / Review)
- ReplayListPage + ReplayPage with ReplayTimeline component
- DashboardPage with MetricCard, range selector, zero-state
- ReviewPage for OpenAPI classification review
- ErrorBanner for WebSocket disconnect handling
- API client (api.ts) with typed fetch wrappers

Infrastructure:
- Frontend Dockerfile (multi-stage node -> nginx)
- nginx.conf with SPA routing + API/WS proxy
- docker-compose.yml with frontend service + healthchecks
- .env.example files (root + backend)

Documentation:
- README.md with quick start and architecture
- Agent configuration guide
- OpenAPI import guide
- Deployment guide
- Demo script

48 new tests, 449 total passing, 92.87% coverage

backend/tests/unit/test_main.py

@@ -13,7 +13,7 @@ class TestMainModule:
         assert app.title == "Smart Support"
 
     def test_app_version(self) -> None:
-        assert app.version == "0.4.0"
+        assert app.version == "0.5.0"
 
     def test_agents_yaml_path_exists(self) -> None:
         assert AGENTS_YAML.name == "agents.yaml"
@@ -33,3 +33,10 @@ class TestMainModule:
     def test_analytics_router_registered(self) -> None:
         routes = [r.path for r in app.routes if hasattr(r, "path")]
         assert any("analytics" in p for p in routes)
+
+    def test_health_route_registered(self) -> None:
+        routes = [r.path for r in app.routes if hasattr(r, "path")]
+        assert "/api/health" in routes
+
+    def test_app_version_is_0_5_0(self) -> None:
+        assert app.version == "0.5.0"