feat: implement super admin and platform admin roles

Add multi-platform admin authorization system with:
- AdminPlatform junction table for admin-platform assignments
- is_super_admin flag on User model for global admin access
- Platform selection flow for platform admins after login
- JWT token updates to include platform context
- New API endpoints for admin user management (super admin only)
- Auth dependencies for super admin and platform access checks

Includes comprehensive test coverage:
- Unit tests for AdminPlatform model and User admin methods
- Unit tests for AdminPlatformService operations
- Integration tests for admin users API endpoints

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/__init__.py

@@ -25,6 +25,7 @@ from fastapi import APIRouter
 
 # Import all admin routers
 from . import (
+    admin_users,
     audit,
     auth,
     background_tasks,
@@ -103,6 +104,9 @@ router.include_router(platforms.router, tags=["admin-platforms"])
 # Include user management endpoints
 router.include_router(users.router, tags=["admin-users"])
 
+# Include admin user management endpoints (super admin only)
+router.include_router(admin_users.router, tags=["admin-admin-users"])
+
 # Include customer management endpoints
 router.include_router(customers.router, tags=["admin-customers"])