feat: RBAC Phase 1 — consolidate user roles into 4-value enum

Consolidate User.role (2-value: admin/store) + User.is_super_admin (boolean)
into a single 4-value UserRole enum: super_admin, platform_admin,
merchant_owner, store_member. Drop stale StoreUser.user_type column.
Fix role="user" bug in merchant creation.

Key changes:
- Expand UserRole enum from 2 to 4 values with computed properties
  (is_admin, is_super_admin, is_platform_admin, is_merchant_owner, is_store_user)
- Add Alembic migration (tenancy_003) for data migration + column drops
- Remove is_super_admin from JWT token payload
- Update all auth dependencies, services, routes, templates, JS, and tests
- Update all RBAC documentation

66 files changed, 1219 unit tests passing.

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

app/modules/tenancy/routes/api/store_auth.py

@@ -77,7 +77,7 @@ def store_login(
     user = login_result["user"]
 
     # CRITICAL: Prevent admin users from using store login
-    if user.role == "admin":
+    if user.is_admin:
         logger.warning(f"Admin user attempted store login: {user.username}")
         raise InvalidCredentialsException(
             "Admins cannot access store portal. Please use admin portal."