Fix canned_responses 500: add missing SQL migration

The canned_responses table was registered in autoMigrate() (app.go) but
autoMigrate() is never called on the serve path (Bootstrap -> NewDatabase
-> RunMigrations only). With no SQL migration file, the table never
existed, so POST /api/v1/accounts/:id/canned_responses failed with
'relation does not exist' -> 500.

Add migration 000049 to create the table matching the GORM model
(id, account_id, content, short_code, timestamps) with the account+
short_code partial unique index and deleted_at index.

Also include two related fixes staged in the working tree:
- router.go: register notifications.GET("") alongside GET("/") to avoid
  Gin 301 trailing-slash redirect that the Vite proxy doesn't follow
- notifications/actions.js: stop the infinite loader on fetch error to
  prevent IntersectionObserver re-fire loop

frontend/app/javascript/dashboard/store/modules/notifications/actions.js

@@ -39,6 +39,10 @@ export const actions = {
       }
     } catch (error) {
       commit(types.SET_NOTIFICATIONS_UI_FLAG, { isFetching: false });
+      // On error, stop the infinite loader — otherwise IntersectionObserver
+      // keeps firing loadMoreNotifications, incrementing page forever and
+      // re-issuing the failing request in a tight loop.
+      commit(types.SET_ALL_NOTIFICATIONS_LOADED);
     }
   },
   unReadCount: async ({ commit } = {}) => {