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/enrichment_service.py

@@ -324,8 +324,7 @@ class EnrichmentService:
             ProspectContact.source_element == "regex",
         ).delete()
 
-        for contact in contacts:
-            db.add(contact)
+        db.add_all(contacts)
 
         # Mark first email and phone as primary
         if contacts: