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>

models/database/__init__.py

@@ -8,6 +8,7 @@ from .admin import (
     AdminSetting,
     PlatformAlert,
 )
+from .admin_platform import AdminPlatform
 from .architecture_scan import (
     ArchitectureScan,
     ArchitectureViolation,
@@ -83,6 +84,7 @@ __all__ = [
     # Admin-specific models
     "AdminAuditLog",
     "AdminNotification",
+    "AdminPlatform",
     "AdminSetting",
     "PlatformAlert",
     "AdminSession",