fix: run DB migrations via migrator service, not standalone app

In Next.js standalone output (output: 'standalone') the src/migrations/
directory is not present on the filesystem, so payload.db.migrate()
could not discover migration files → home_page table was never created
→ getHomePage() threw a DB error → Server Components render failed.

Fix:
- Add `migrator` service to docker-compose.prod.yml (builds from the
  `migrator` Dockerfile stage which has the full src/ tree)
- migrator runs `pnpm payload migrate` before app starts
- app depends_on migrator: service_completed_successfully
- Remove payload.db.migrate() from instrumentation.ts (migrator handles it)

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

docker-compose.prod.yml

@@ -1,4 +1,19 @@
 services:
+  # Run pending Payload migrations before the app starts.
+  # Uses the `migrator` image stage (has full src/) so TS files are discoverable.
+  migrator:
+    build:
+      context: .
+      target: migrator
+    restart: "no"
+    networks:
+      - internal
+    env_file:
+      - .env.production
+    depends_on:
+      db:
+        condition: service_healthy
+
   app:
     build:
       context: .
@@ -20,6 +35,8 @@ services:
     depends_on:
       db:
         condition: service_healthy
+      migrator:
+        condition: service_completed_successfully
 
   db:
     image: postgres:17-alpine

src/instrumentation.ts

@@ -1,17 +1,11 @@
 // Runs on Next.js startup (Node.js runtime only) before any requests are handled.
-// Uses compiled/bundled code — no tsx/ESM resolution issues.
+// Migrations are handled by the `migrator` Docker service (docker-compose.prod.yml)
+// which runs before this container starts, so no migration call is needed here.
 export async function register() {
   if (process.env.NEXT_RUNTIME === 'nodejs') {
     const { getPayload } = await import('payload');
     const { default: config } = await import('@payload-config');
-
-    const payload = await getPayload({ config });
-
-    // In dev mode Payload auto-pushes schema changes — migrations only needed in production
-    if (process.env.NODE_ENV === 'production') {
-      console.log('[startup] Running database migrations...');
-      await payload.db.migrate();
-      console.log('[startup] Migrations complete.');
-    }
+    // Initialize Payload (connects to DB, registers collections/globals)
+    await getPayload({ config });
   }
 }