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/campaign_service.py

@@ -63,8 +63,7 @@ class CampaignService:
             is_active=data.get("is_active", True),
         )
         db.add(template)
-        db.commit()
-        db.refresh(template)
+        db.flush()
         return template
 
     def update_template(self, db: Session, template_id: int, data: dict) -> CampaignTemplate:
@@ -72,14 +71,13 @@ class CampaignService:
         for field in ["name", "lead_type", "channel", "language", "subject_template", "body_template", "is_active"]:
             if field in data and data[field] is not None:
                 setattr(template, field, data[field])
-        db.commit()
-        db.refresh(template)
+        db.flush()
         return template
 
     def delete_template(self, db: Session, template_id: int) -> bool:
         template = self.get_template_by_id(db, template_id)
         db.delete(template)
-        db.commit()
+        db.flush()
         return True
 
     # --- Rendering ---
@@ -143,7 +141,7 @@ class CampaignService:
             db.add(send)
             sends.append(send)
 
-        db.commit()
+        db.flush()
         logger.info("Sent campaign %d to %d prospects", template_id, len(prospect_ids))
         return sends