feat: mentions badge in chat list and muted-mention delivery

app/chats/repository.py

@@ -237,6 +237,42 @@ async def get_unread_count_for_chat(db: AsyncSession, *, chat_id: int, user_id:
     return int(result.scalar_one() or 0)
 
 
+async def get_unread_mentions_count_for_chat(
+    db: AsyncSession,
+    *,
+    chat_id: int,
+    user_id: int,
+    username: str | None,
+) -> int:
+    normalized_username = (username or "").strip().lower()
+    if not normalized_username:
+        return 0
+    last_read_subquery = (
+        select(MessageReceipt.last_read_message_id)
+        .where(MessageReceipt.chat_id == chat_id, MessageReceipt.user_id == user_id)
+        .limit(1)
+        .scalar_subquery()
+    )
+    mention_like = f"%@{normalized_username}%"
+    stmt = (
+        select(func.count(Message.id))
+        .outerjoin(
+            MessageHidden,
+            (MessageHidden.message_id == Message.id) & (MessageHidden.user_id == user_id),
+        )
+        .where(
+            Message.chat_id == chat_id,
+            Message.sender_id != user_id,
+            MessageHidden.id.is_(None),
+            Message.id > func.coalesce(last_read_subquery, 0),
+            Message.text.is_not(None),
+            func.lower(Message.text).like(mention_like),
+        )
+    )
+    result = await db.execute(stmt)
+    return int(result.scalar_one() or 0)
+
+
 async def get_last_visible_message_for_user(
     db: AsyncSession,
     *,