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/integration/test_websocket.py

@@ -315,7 +315,7 @@ class TestWebSocketValidation:
     @pytest.mark.asyncio
     async def test_content_too_long(self) -> None:
         g, sm, im, cb, ws = _setup()
-        raw = json.dumps({"type": "message", "thread_id": "t1", "content": "x" * 9000})
+        raw = json.dumps({"type": "message", "thread_id": "t1", "content": "x" * 10001})
         await dispatch_message(ws, g, sm, cb, raw, interrupt_manager=im)
         assert ws.sent[0]["type"] == "error"
         assert "too long" in ws.sent[0]["message"].lower()