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

CLAUDE.md

@@ -241,7 +241,7 @@ A checkpoint includes:
 | 2 | `phase-2/multi-agent-safety` | Supervisor routing + interrupts + templates | COMPLETED (2026-03-30) |
 | 3 | `phase-3/openapi-discovery` | OpenAPI parsing + MCP generation + SSRF protection | COMPLETED (2026-03-30) |
 | 4 | `phase-4/analytics-replay` | Replay API + analytics dashboard | COMPLETED (2026-03-31) |
-| 5 | `phase-5/polish-demo` | Error hardening + demo prep + Docker deploy | NOT STARTED |
+| 5 | `phase-5/polish-demo` | Error hardening + demo prep + Docker deploy | COMPLETED (2026-03-31) |
 
 Status values: `NOT STARTED` -> `IN PROGRESS` -> `COMPLETED (YYYY-MM-DD)`