feat: add message reliability foundation

- implement idempotent message creation via client_message_id

- add persistent delivered/read receipts

- expose /messages/status and wire websocket receipt events

- update web client to send client ids and auto-ack delivered/read

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
+from app.messages.models import Message, MessageIdempotencyKey, MessageReceipt
 from app.notifications.models import NotificationLog
 from app.users.models import User
 
@@ -13,6 +13,8 @@ __all__ = [
     "EmailLog",
     "EmailVerificationToken",
     "Message",
+    "MessageIdempotencyKey",
+    "MessageReceipt",
     "NotificationLog",
     "PasswordResetToken",
     "User",