Fix stale DB session after long embedding pre-load

The embedding pre-load for 4998 existing controls takes ~16 minutes,
causing the SQLAlchemy session to become invalid. Added rollback after
pre-load completes to reset the session before subsequent DB operations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend-compliance/compliance/services/control_generator.py

@@ -1391,6 +1391,12 @@ Gib ein JSON-Array zurueck mit GENAU {len(chunks)} Objekten. Jedes Objekt hat di
         loaded = sum(1 for emb in embeddings if emb)
         logger.info("Pre-loaded %d/%d embeddings", loaded, len(texts))
 
+        # Reset DB session after long-running embedding operation to avoid stale connections
+        try:
+            self.db.rollback()
+        except Exception:
+            pass
+
     def _load_existing_controls(self) -> list[dict]:
         """Load existing controls from DB (cached per pipeline run)."""
         if self._existing_controls is not None: