feat(privacy): user blocklist with private chat enforcement

- add blocked_users table and migration
- add users API: block, unblock, list blocked users
- prevent private chat creation and private messaging when block relation exists
- add block/unblock action in private chat info panel

app/users/service.py

@@ -52,3 +52,21 @@ async def update_user_profile(
     await db.commit()
     await db.refresh(user)
     return user
+
+
+async def block_user(db: AsyncSession, *, user_id: int, blocked_user_id: int) -> None:
+    await repository.block_user(db, user_id=user_id, blocked_user_id=blocked_user_id)
+    await db.commit()
+
+
+async def unblock_user(db: AsyncSession, *, user_id: int, blocked_user_id: int) -> None:
+    await repository.unblock_user(db, user_id=user_id, blocked_user_id=blocked_user_id)
+    await db.commit()
+
+
+async def has_block_relation_between_users(db: AsyncSession, *, user_a_id: int, user_b_id: int) -> bool:
+    return await repository.has_block_relation_between_users(db, user_a_id=user_a_id, user_b_id=user_b_id)
+
+
+async def list_blocked_users(db: AsyncSession, *, user_id: int) -> list[User]:
+    return await repository.list_blocked_users(db, user_id=user_id)