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/models/base.py

@@ -0,0 +1,9 @@
+"""SQLAlchemy declarative base — importable without triggering engine creation."""
+
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    """Base class for all SQLAlchemy ORM models."""
+
+    pass