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/routes/api/admin_prospects.py

@@ -82,6 +82,7 @@ def create_prospect(
         data.model_dump(exclude_none=True),
         captured_by_user_id=current_admin.user_id,
     )
+    db.commit()
     return _to_response(prospect)
 
 
@@ -94,6 +95,7 @@ def update_prospect(
 ):
     """Update a prospect."""
     prospect = prospect_service.update(db, prospect_id, data.model_dump(exclude_none=True))
+    db.commit()
     return _to_response(prospect)
 
 
@@ -105,6 +107,7 @@ def delete_prospect(
 ):
     """Delete a prospect."""
     prospect_service.delete(db, prospect_id)
+    db.commit()
     return ProspectDeleteResponse(message="Prospect deleted")
 
 
@@ -125,6 +128,7 @@ async def import_domains(
             domains.append(domain)
 
     created, skipped = prospect_service.create_bulk(db, domains, source="csv_import")
+    db.commit()
     return ProspectImportResponse(created=created, skipped=skipped, total=len(domains))