Add search-server target and configurable search mode

- Add SearchServer module target for standalone search service
- Add search_mode config: "", "embedded" (default), "remote"
- Add search_server_address config for remote search server
- Create remote_search.go: gRPC client wrapper for remote search
- Create search_service.go: standalone search gRPC service
- Modify service.go: conditional search mode handling
- Backward compatible: empty/embedded mode works as before

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/setting/setting.go

@@ -623,6 +623,8 @@ type Cfg struct {
 	OverridesFilePath                          string
 	OverridesReloadInterval                    time.Duration
 	EnableSQLKVBackend                         bool
+	SearchMode                                 string // "", "embedded", "remote" - empty defaults to embedded
+	SearchServerAddress                        string // gRPC address for remote search server
 
 	// Secrets Management
 	SecretsManagement SecretsManagerSettings