feat: add unique constraints for custom subdomain and domain per platform

Add UNIQUE(custom_subdomain, platform_id) on store_platforms to prevent
two stores from claiming the same subdomain on the same platform.
Add UNIQUE(store_id, platform_id) on store_domains to enforce one custom
domain per store per platform.

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

app/modules/tenancy/models/store_platform.py

@@ -152,6 +152,12 @@ class StorePlatform(Base, TimestampMixin):
             "platform_id",
             name="uq_store_platform",
         ),
+        # Same custom_subdomain cannot be claimed twice on the same platform
+        UniqueConstraint(
+            "custom_subdomain",
+            "platform_id",
+            name="uq_custom_subdomain_platform",
+        ),
         # Performance indexes
         Index(
             "idx_store_platform_active",