feat: add role-based admin support for multiple admin users

- Add Role model and user_roles association table (many-to-many)
- Add password_hash column to User model (nullable, for future bcrypt)
- Keep is_admin boolean for backward compatibility
- Add is_admin_effective property: true if legacy is_admin OR 'admin' role
- Update auth dependency (get_current_admin_user) to use is_admin_effective
- Update bootstrap login to auto-create 'admin' role and assign on login
- Update Google OAuth login to use is_admin_effective for token creation
- Add migrate_roles.py: idempotent migration script to backfill roles
- Add unit tests for Role, User, association table, and is_admin_effective

backend/app/auth.py

@@ -98,8 +98,11 @@ async def get_current_user(
 async def get_current_admin_user(
     current_user: User = Depends(get_current_user),
 ) -> User:
-    """Raise 403 if the current user is not an admin."""
-    if not current_user.is_admin:
+    """Raise 403 if the current user is not an admin.
+
+    Checks both the legacy is_admin flag and the new role-based admin role.
+    """
+    if not current_user.is_admin_effective:
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Admin access required")
     return current_user