feat(templates): source templates for multi-key balancing

A template stores every source field except name and api_key, so
operators spin up N key-bearing sources from one shared skeleton
instead of duplicating the whole source block N times.

- config: SourceTemplate type + RuntimeConfig.SourceTemplates stored
  in runtime.json alongside runtime sources
- store: UpsertTemplate / ListTemplates / RemoveTemplate
- core: Templates / SaveTemplate / RemoveTemplate
- gateway: GET/POST/DELETE /api/source_templates
- webui: source list gains a Templates button opening a manager with
  per-template edit/delete; the add-source dialog gains 'from
  template' (event-delegated picker) and 'as template' (card modal)
  buttons in its header; z-index fixed so the template editor layers
  above the manager

internal/gateway/server.go

@@ -218,6 +218,8 @@ func (g *Gateway) routes(w http.ResponseWriter, r *http.Request) {
 		g.handleAdaptersAPI(w, r)
 	case r.URL.Path == "/api/sources" || strings.HasPrefix(r.URL.Path, "/api/sources/"):
 		g.handleSourcesAPI(w, r)
+	case r.URL.Path == "/api/source_templates" || strings.HasPrefix(r.URL.Path, "/api/source_templates/"):
+		g.handleSourceTemplatesAPI(w, r)
 	case r.URL.Path == "/api/chat":
 		g.handleChat(w, r)
 	case r.URL.Path == "/api/status":