feat(middleware): harden routing with fail-closed policy, custom subdomain management, and perf fixes

- Fix IPv6 host parsing with _strip_port() utility
- Remove dangerous StorePlatform→Store.subdomain silent fallback
- Close storefront gate bypass when frontend_type is None
- Add custom subdomain management UI and API for stores
- Add domain health diagnostic tool
- Convert db.add() in loops to db.add_all() (24 PERF-006 fixes)
- Add tests for all new functionality (18 subdomain service tests)
- Add .github templates for validator compliance

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

app/modules/prospecting/services/campaign_service.py

@@ -138,9 +138,9 @@ class CampaignService:
                 sent_at=datetime.now(UTC),
                 sent_by_user_id=sent_by_user_id,
             )
-            db.add(send)
             sends.append(send)
 
+        db.add_all(sends)
         db.flush()
         logger.info("Sent campaign %d to %d prospects", template_id, len(prospect_ids))
         return sends