feat: hot-reload aibridged and aibridgeproxyd providers on DB changes (#25673)

Previously the in-process aibridge daemon and the enterprise aibridgeproxy daemon both snapshotted their provider routing once at boot. Any `ai_providers` or `ai_provider_keys` mutation required a restart for either to pick it up.

Add an `ai_providers_changed` pubsub channel that the CRUD handlers publish on after Create / Update / Delete. Both daemons subscribe:

- **aibridged** rebuilds its `[]aibridge.Provider` snapshot via `BuildProviders` and swaps it into the pool atomically. Inflight requests keep serving against the bridge they already acquired; new acquires build against the new snapshot. Per-provider construction errors stay scoped to the offending row.
- **aibridgeproxyd** rebuilds its routing snapshot from `GetAIProviders` and swaps the host→provider map atomically. The MITM listener picks up new providers without restart.

DB read for aibridgeproxyd uses the existing `AsAIProviderMetadataReader` subject for routing-only access.

cli/server.go

@@ -1046,7 +1046,8 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 				if err != nil {
 					return xerrors.Errorf("build AI providers: %w", err)
 				}
-				aibridgeDaemon, err = newAIBridgeDaemon(coderAPI, aibridgeProviders)
+				var unsubscribeProviderReload func()
+				aibridgeDaemon, unsubscribeProviderReload, err = newAIBridgeDaemon(coderAPI, aibridgeProviders, vals.AI.BridgeConfig)
 				if err != nil {
 					return xerrors.Errorf("create aibridged: %w", err)
 				}
@@ -1055,6 +1056,7 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 				// daemon does not affect in-flight requests but is needed to
 				// release pool/recorder resources at shutdown.
 				defer aibridgeDaemon.Close()
+				defer unsubscribeProviderReload()
 			}
 
 			if vals.Prometheus.Enable {