refactor(prospecting): migrate SVC-006 transaction control to endpoint level

Move db.commit() from services to API endpoints and Celery tasks.
Services now use db.flush() only; endpoints own the transaction.

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

app/modules/prospecting/services/interaction_service.py

@@ -38,8 +38,7 @@ class InteractionService:
             created_by_user_id=user_id,
         )
         db.add(interaction)
-        db.commit()
-        db.refresh(interaction)
+        db.flush()
         logger.info("Interaction logged for prospect %d: %s", prospect_id, data["interaction_type"])
         return interaction