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>

alembic/versions/z_add_unique_subdomain_domain_constraints.py

@@ -0,0 +1,33 @@
+"""add unique constraints for custom_subdomain and store domain per platform
+
+Revision ID: z_unique_subdomain_domain
+Revises: a44f4956cfb1
+Create Date: 2026-02-26
+"""
+from alembic import op
+
+revision = "z_unique_subdomain_domain"
+down_revision = ("a44f4956cfb1", "tenancy_003")
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # StorePlatform: same custom_subdomain cannot be claimed twice on the same platform
+    op.create_unique_constraint(
+        "uq_custom_subdomain_platform",
+        "store_platforms",
+        ["custom_subdomain", "platform_id"],
+    )
+
+    # StoreDomain: a store can have at most one custom domain per platform
+    op.create_unique_constraint(
+        "uq_store_domain_platform",
+        "store_domains",
+        ["store_id", "platform_id"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("uq_store_domain_platform", "store_domains", type_="unique")
+    op.drop_constraint("uq_custom_subdomain_platform", "store_platforms", type_="unique")