feat(notifications): per-chat mute settings

- add chat_notification_settings table and migration
- add chat notifications API (get/update muted)
- skip message notifications for muted recipients
- add mute/unmute control in chat info panel

app/chats/schemas.py

@@ -74,3 +74,13 @@ class ChatDeleteRequest(BaseModel):
 
 class ChatDiscoverRead(ChatRead):
     is_member: bool
+
+
+class ChatNotificationSettingsRead(BaseModel):
+    chat_id: int
+    user_id: int
+    muted: bool
+
+
+class ChatNotificationSettingsUpdate(BaseModel):
+    muted: bool