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/models/admin_platform.py

@@ -3,8 +3,8 @@
 AdminPlatform junction table for many-to-many relationship between Admin Users and Platforms.
 
 This enables platform-scoped admin access:
-- Super Admins: Have is_super_admin=True on User model, bypass this table
-- Platform Admins: Assigned to specific platforms via this junction table
+- Super Admins: Have role='super_admin' on User model, bypass this table
+- Platform Admins: Have role='platform_admin', assigned to specific platforms via this junction table
 
 A platform admin CAN be assigned to multiple platforms (e.g., both OMS and Loyalty).
 """