feat(observability): phase 1 - schema, models & event log backend

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

internal/api/sse.go

@@ -150,9 +150,9 @@ func (s *Server) streamEvents(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusOK)
 	flusher.Flush()
 
-	// Subscribe to instance status and deploy status events.
+	// Subscribe to instance status, deploy status, and persistent event log events.
 	sub := s.eventBus.Subscribe(func(evt events.Event) bool {
-		return evt.Type == events.EventInstanceStatus || evt.Type == events.EventDeployStatus
+		return evt.Type == events.EventInstanceStatus || evt.Type == events.EventDeployStatus || evt.Type == events.EventLog
 	})
 	defer s.eventBus.Unsubscribe(sub)