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>

tests/fixtures/admin_platform_fixtures.py

@@ -78,10 +78,9 @@ def platform_admin_with_platform(db, auth_manager, test_platform, test_super_adm
         email=f"padmin_{unique_id}@example.com",
         username=f"padmin_{unique_id}",
         hashed_password=hashed_password,
-        role="admin",
+        role="platform_admin",
         is_active=True,
         is_email_verified=True,
-        is_super_admin=False,
     )
     db.add(admin)
     db.flush()