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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+## Summary
+
+<!-- Brief description of what this PR does -->
+
+## Changes
+
+-
+
+## Test plan
+
+- [ ] Unit tests pass (`python -m pytest tests/unit/`)
+- [ ] Integration tests pass (`python -m pytest tests/integration/`)
+- [ ] Architecture validation passes (`python scripts/validate/validate_all.py`)
+
+## Checklist
+
+- [ ] Code follows project conventions
+- [ ] No new warnings introduced
+- [ ] Database migrations included (if applicable)