feat(core): clear saved chat and add message deletion scopes

backend:

- add message_hidden table for per-user message hiding

- support DELETE /messages/{id}?for_all=true|false

- implement delete-for-me vs delete-for-all logic by chat type/permissions

- add POST /chats/{chat_id}/clear and route saved chat deletion to clear

web:

- saved messages action changed from delete to clear

- message context menu now supports delete modal: for me / for everyone

- add local store helpers removeMessage/clearChatMessages

- include realtime stability improvements and app error boundary

app/database/models.py

@@ -2,7 +2,7 @@ from app.auth.models import EmailVerificationToken, PasswordResetToken
 from app.chats.models import Chat, ChatMember
 from app.email.models import EmailLog
 from app.media.models import Attachment
-from app.messages.models import Message, MessageIdempotencyKey, MessageReceipt
+from app.messages.models import Message, MessageHidden, MessageIdempotencyKey, MessageReceipt
 from app.notifications.models import NotificationLog
 from app.users.models import User