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/prospect_service.py

@@ -137,8 +137,7 @@ class ProspectService:
             )
             db.add(contact)
 
-        db.commit()
-        db.refresh(prospect)
+        db.flush()
         logger.info("Created prospect: %s (channel=%s)", prospect.display_name, channel)
         return prospect
 
@@ -161,7 +160,7 @@ class ProspectService:
             db.add(prospect)
             created += 1
 
-        db.commit()
+        db.flush()
         logger.info("Bulk import: %d created, %d skipped", created, skipped)
         return created, skipped
 
@@ -178,14 +177,13 @@ class ProspectService:
                 tags = json.dumps(tags)
             prospect.tags = tags
 
-        db.commit()
-        db.refresh(prospect)
+        db.flush()
         return prospect
 
     def delete(self, db: Session, prospect_id: int) -> bool:
         prospect = self.get_by_id(db, prospect_id)
         db.delete(prospect)
-        db.commit()
+        db.flush()
         logger.info("Deleted prospect: %d", prospect_id)
         return True