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_domain.py

@@ -68,6 +68,8 @@ class StoreDomain(Base, TimestampMixin):
     # Constraints
     __table_args__ = (
         UniqueConstraint("store_id", "domain", name="uq_store_domain"),
+        # A store can have at most one custom domain per platform
+        UniqueConstraint("store_id", "platform_id", name="uq_store_domain_platform"),
         Index("idx_domain_active", "domain", "is_active"),
         Index("idx_store_domain_primary", "store_id", "is_primary"),
         Index("idx_store_domain_platform", "platform_id"),