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/router.go

@@ -125,6 +125,8 @@ func (s *Server) Router() chi.Router {
 			r.Get("/deploys", s.listDeploys)
 			r.Get("/deploys/{id}/logs", s.streamDeployLogs)
 			r.Get("/events", s.streamEvents)
+			r.Get("/events/log", s.listEventLog)
+			r.Get("/events/log/stats", s.getEventLogStats)
 			r.Get("/registries", s.listRegistries)
 			r.Route("/registries/{id}", func(r chi.Router) {
 				r.Get("/tags/*", s.listRegistryTags)