fix(backend): use generated import extension

packages/backend/convex/migrations.ts

@@ -1,6 +1,6 @@
 import { v } from 'convex/values';
 
-import { internalMutation } from './_generated/server';
+import { internalMutation } from './_generated/server.js';
 
 // One-shot backfill: preserve existing automatic fast-forward behavior for
 // GitHub spoons now that refreshes honor autoSyncEnabled. Idempotent.