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

cmd/server/main.go

@@ -93,6 +93,21 @@ func main() {
 	notifier := notify.New()
 	eventBus := events.New()
 
+	// Auto-persist warn/error events from the event bus to the database.
+	stopLogger := eventBus.RegisterPersistentLogger(func(source, severity, message, metadata string) (int64, string, error) {
+		evt, err := db.InsertEvent(store.EventLog{
+			Source:   source,
+			Severity: severity,
+			Message:  message,
+			Metadata: metadata,
+		})
+		if err != nil {
+			return 0, "", err
+		}
+		return evt.ID, evt.CreatedAt, nil
+	})
+	defer stopLogger()
+
 	dep := deployer.New(dockerClient, npmClient, db, healthChecker, notifier, eventBus, encKey)
 
 	// Initialize webhook handler.