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/billing/tests/unit/test_billing_metrics.py

@@ -83,13 +83,12 @@ def billing_extra_platforms(db):
     """Create additional platforms for multiple subscriptions (unique constraint: merchant+platform)."""
     platforms = []
     for i in range(2):
-        p = Platform(
+        platforms.append(Platform(
             code=f"bm_extra_{uuid.uuid4().hex[:8]}",
             name=f"Extra Platform {i}",
             is_active=True,
-        )
-        db.add(p)
-        platforms.append(p)
+        ))
+    db.add_all(platforms)
     db.commit()
     for p in platforms:
         db.refresh(p)

app/modules/billing/tests/unit/test_feature_service.py

@@ -90,8 +90,7 @@ def fs_tier_with_features(db, fs_tier):
         TierFeatureLimit(tier_id=fs_tier.id, feature_code="feature_b", limit_value=100),
         TierFeatureLimit(tier_id=fs_tier.id, feature_code="feature_c", limit_value=50),
     ]
-    for f in features:
-        db.add(f)
+    db.add_all(features)
     db.commit()
     return features