Critical fixes:
- Fix StaleContainer frontend type to match nested backend response shape
- Guard ContainerID[:12] slice against empty/short IDs in ListAllProxies
High-priority fixes:
- Support comma-separated severity/source in event log filtering (IN clause)
- Eliminate N+1 queries in ListAllProxies and FindStaleInstances (pre-load maps)
- Stop leaking internal error messages to API clients (use slog + generic msgs)
Add database foundation for observability features:
- event_log table with severity/source filtering and pagination
- standalone_proxies table for user-created reverse proxies
- stale_threshold_days setting (default 7 days)
- Auto-persist warn/error events from event bus to database
- SSE broadcast of persistent events for real-time UI updates
- Frontend types and API functions for downstream UI phases
alexei.dolgolyov