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>

tests/conftest.py

@@ -158,6 +158,7 @@ def cleanup():
 
 # Import fixtures from fixture modules
 pytest_plugins = [
+    "tests.fixtures.admin_platform_fixtures",
     "tests.fixtures.auth_fixtures",
     "tests.fixtures.marketplace_product_fixtures",
     "tests.fixtures.vendor_fixtures",