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/api_key.py

@@ -8,7 +8,7 @@ from sqlalchemy import DateTime, ForeignKey, Text, func
 from sqlalchemy.dialects.postgresql import JSONB, UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
-from app.database import Base
+from app.models.base import Base
 
 
 class ApiKey(Base):