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>

.github/workflows/ci.yml

@@ -47,6 +47,7 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v6
         with:
+          version: latest
           working-directory: poller
 
   frontend-lint: