feat(api): add /api/admin/lidarr/* config + profiles + folders + test

Five admin-only handlers under RequireAdmin middleware: GET/PUT config
(api_key masked, empty key on PUT preserves saved), POST test (always
200, maps Lidarr errors to stable codes), GET quality-profiles, GET
root-folders. 10 HTTP integration tests, all green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/api.go

@@ -60,6 +60,15 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Get("/requests", h.handleListRequests)
 			authed.Get("/requests/{id}", h.handleGetRequest)
 			authed.Delete("/requests/{id}", h.handleCancelRequest)
+
+			authed.Route("/admin", func(admin chi.Router) {
+				admin.Use(auth.RequireAdmin())
+				admin.Get("/lidarr/config", h.handleGetLidarrConfig)
+				admin.Put("/lidarr/config", h.handlePutLidarrConfig)
+				admin.Post("/lidarr/test", h.handleTestLidarrConnection)
+				admin.Get("/lidarr/quality-profiles", h.handleListQualityProfiles)
+				admin.Get("/lidarr/root-folders", h.handleListRootFolders)
+			})
 		})
 	})
 }