feat: robust automatic DB migration for Docker deployments

Backup before migration (pg_dump/SQLite copy), DB connection wait with
retries, idempotent prisma migrate deploy, old backup cleanup (keep 5),
and server refuses to start if migration fails.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/docker-compose.yml

@@ -51,6 +51,8 @@ services:
     volumes:
       # Persist uploaded images and files
       - keep-uploads:/app/data/uploads
+      # Persist migration backups
+      - backup-data:/app/data/backups
 
       # Optional: Mount custom SSL certificates
       # - ./certs:/app/certs:ro
@@ -106,5 +108,7 @@ volumes:
     driver: local
   keep-uploads:
     driver: local
+  backup-data:
+    driver: local
   # ollama-data:
   #   driver: local