feat(api): add feature flag management endpoints

Add FeatureHandler to manage lightweight runtime configuration toggles
stored in the Redis `meta:features` hash. This enables dynamic control
over system behavior (e.g., storage backends) via the admin API.

- Add `GET /admin/features` to list flags
- Add `PUT /admin/features` to update flags
- Update README with feature flag documentation

cmd/server/main.go

@@ -113,6 +113,7 @@ func main() {
 	handler := api.NewHandler(db, syncService, logWriter)
 	adminHandler := api.NewAdminHandler(masterService, syncService)
 	masterHandler := api.NewMasterHandler(masterService, syncService)
+	featureHandler := api.NewFeatureHandler(rdb)
 
 	// 4.1 Prime Redis snapshots so DP can start with data
 	if err := syncService.SyncAll(db); err != nil {
@@ -157,6 +158,8 @@ func main() {
 	adminGroup.Use(middleware.AdminAuthMiddleware(adminService))
 	{
 		adminGroup.POST("/masters", adminHandler.CreateMaster)
+		adminGroup.GET("/features", featureHandler.ListFeatures)
+		adminGroup.PUT("/features", featureHandler.UpdateFeatures)
 		// Other admin routes for managing providers, models, etc.
 		adminGroup.POST("/providers", handler.CreateProvider)
 		adminGroup.POST("/models", handler.CreateModel)