feat: add migration support for VPS deployment with backup functionality

docs/vps-deployment.md

@@ -120,6 +120,34 @@ If the Caddy container also needs to be recreated:
 DEPLOY_CADDY=1 pnpm deploy:vps
 ```
 
+## Payload Migrations
+
+Routine deploys do not run migrations. For schema-changing commits, create and
+commit the Payload migration files, then deploy with migrations enabled:
+
+```bash
+RUN_MIGRATIONS=1 pnpm deploy:vps
+```
+
+That mode:
+
+1. syncs the committed code
+2. builds the new Docker image
+3. stops the app container
+4. copies `shared/content.db` to `backups/content-*-pre-migrate.db`
+5. runs `pnpm payload migrate` in a one-off app container
+6. starts the app container again
+
+To run migrations against the already deployed code/image without syncing code:
+
+```bash
+pnpm vps:migrate
+```
+
+Run migrations only when the commit includes migration files under
+`src/migrations/`. For plain frontend/content-rendering changes, use the normal
+`pnpm deploy:vps` path.
+
 ## Status And Logs
 
 ```bash