feat(subscriptions): migrate subscription management to merchant level and seed tiers

Move subscription create/edit from store detail (broken endpoint) to merchant
detail page with proper modal UI. Seed 4 subscription tiers (Essential,
Professional, Business, Enterprise) in init_production.py. Also includes
cross-module dependency declarations, store domain platform_id migration,
platform context middleware, CMS route fixes, and migration backups.

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

app/modules/tenancy/services/store_domain_service.py

@@ -101,11 +101,23 @@ class StoreDomainService:
             if domain_data.is_primary:
                 self._unset_primary_domains(db, store_id)
 
+            # Resolve platform_id: use provided value, or auto-resolve from primary StorePlatform
+            platform_id = domain_data.platform_id
+            if not platform_id:
+                from app.modules.tenancy.models import StorePlatform
+                primary_sp = (
+                    db.query(StorePlatform)
+                    .filter(StorePlatform.store_id == store_id, StorePlatform.is_primary.is_(True))
+                    .first()
+                )
+                platform_id = primary_sp.platform_id if primary_sp else None
+
             # Create domain record
             new_domain = StoreDomain(
                 store_id=store_id,
                 domain=normalized_domain,
                 is_primary=domain_data.is_primary,
+                platform_id=platform_id,
                 verification_token=secrets.token_urlsafe(32),
                 is_verified=False,  # Requires DNS verification
                 is_active=False,  # Cannot be active until verified