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/migrate_roles.py

@@ -0,0 +1,60 @@
+"""
+Migration seed: create the 'admin' role and assign it to existing is_admin=True users.
+
+Run once after deploying the updated schema. Idempotent — safe to run multiple times.
+
+Usage:
+    KMTN_DATABASE_URL=... python migrate_roles.py
+    (run from the backend/ directory)
+"""
+
+import asyncio
+from sqlalchemy import select, text
+
+from app.database import async_session
+from app.user_models import User, Role, user_roles
+
+
+async def migrate() -> None:
+    """Ensure 'admin' role exists and assign to existing is_admin=True users."""
+    async with async_session() as session:
+        # 1) Create 'admin' role if it doesn't exist
+        result = await session.execute(select(Role).where(Role.name == "admin"))
+        admin_role = result.scalar_one_or_none()
+        if admin_role is None:
+            admin_role = Role(name="admin", description="Full administrative access")
+            session.add(admin_role)
+            await session.commit()
+            await session.refresh(admin_role)
+            print(f"  ✓ Created 'admin' role (id={admin_role.id})")
+        else:
+            print(f"  ✓ 'admin' role already exists (id={admin_role.id})")
+
+        # 2) Assign 'admin' role to all existing is_admin=True users
+        result = await session.execute(
+            select(User).where(User.is_admin == True)  # noqa: E712
+        )
+        admin_users = result.scalars().all()
+        migrated = 0
+        for user in admin_users:
+            if admin_role not in user.roles:
+                user.roles.append(admin_role)
+                migrated += 1
+        if migrated:
+            await session.commit()
+            print(f"  ✓ Assigned 'admin' role to {migrated} existing admin user(s)")
+        else:
+            print("  ✓ No new role assignments needed")
+
+        # 3) Summary
+        result = await session.execute(
+            select(User).where(user_roles.c.role_id == admin_role.id)
+        )
+        role_admins = result.scalars().all()
+        print(f"  → {len(role_admins)} user(s) now have the 'admin' role")
+
+
+if __name__ == "__main__":
+    print("Running role migration...")
+    asyncio.run(migrate())
+    print("Done.")