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>

mkdocs.yml

@@ -147,6 +147,8 @@ nav:
         - Developer Documentation: development/error-rendering/error-rendering-developer-documentation.md
         - Flow Diagram: development/error-rendering/html-error-rendering-flow-diagram.md
       - Environment Detection: development/environment-detection.md
+      - Diagnostics:
+        - Domain Health: development/diagnostics/domain-health.md
       - Database Seeder:
         - Documentation: development/database-seeder/database-seeder-documentation.md
         - Makefile Guide: development/database-seeder/makefile-database-seeder.md