refactor: engineering improvements -- API versioning, structured logging, Alembic, error standardization, test coverage

- API versioning: all REST endpoints prefixed with /api/v1/
- Structured logging: replaced stdlib logging with structlog (console/JSON modes)
- Alembic migrations: versioned DB schema with initial migration
- Error standardization: global exception handlers for consistent envelope format
- Interrupt cleanup: asyncio background task for expired interrupt removal
- Integration tests: +30 tests (analytics, replay, openapi, error, session APIs)
- Frontend tests: +57 tests (all components, pages, useWebSocket hook)
- Backend: 557 tests, 89.75% coverage | Frontend: 80 tests, 16 test files

.env.example

@@ -26,6 +26,10 @@ WEBHOOK_URL=
 SESSION_TTL_MINUTES=30
 INTERRUPT_TTL_MINUTES=30
 
+# Optional: API key for admin endpoints (analytics, replay, openapi, websocket)
+# Leave empty to disable authentication (dev mode)
+ADMIN_API_KEY=
+
 # Optional: load a named agent template instead of agents.yaml
 # Available templates: ecommerce, saas, generic
 TEMPLATE_NAME=