fix: legacy DB support + QA audit fixes

- Entrypoint: for legacy DBs, add is_admin via raw SQL then stamp head
  (instead of running full migration chain which fails on existing tables)
- Fixed admin_api_keys.py Pydantic config (dict → ConfigDict)
- Added tests/test_alembic_migrations.py (PostgreSQL-only integration tests)
- Fixed infinite recursion in _create_table_if_not_exists (QA catch)

backend/src/api/routes/admin_api_keys.py

@@ -11,7 +11,7 @@
 from datetime import datetime
 
 from fastapi import APIRouter, Depends, HTTPException, status
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 from sqlalchemy.orm import Session
 
 from ...core.auth.api_key import generate_api_key
@@ -62,7 +62,7 @@ class ApiKeyListItem(BaseModel):
     expires_at: datetime | None
     last_used_at: datetime | None
 
-    model_config = {"from_attributes": True}
+    model_config = ConfigDict(from_attributes=True)
 # #endregion ApiKeyListItem