fix(ci): fix alembic DB import and golangci-lint version

- Move Base to app/models/base.py so alembic env.py can import it
  without triggering engine creation (which connects to hardcoded DB)
- Update all 13 models to import Base from app.models.base
- Pin golangci-lint to latest (supports Go 1.25)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/database.py

@@ -10,15 +10,8 @@ from sqlalchemy.ext.asyncio import (
     async_sessionmaker,
     create_async_engine,
 )
-from sqlalchemy.orm import DeclarativeBase
-
 from app.config import settings
-
-
-class Base(DeclarativeBase):
-    """Base class for all SQLAlchemy ORM models."""
-
-    pass
+from app.models.base import Base  # noqa: F401 — re-exported for backwards compat
 
 
 # Primary engine using postgres superuser (for migrations/admin)