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/messages/schemas.py

@@ -1,4 +1,5 @@
 from datetime import datetime
+from typing import Literal
 
 from pydantic import BaseModel, ConfigDict, Field
 
@@ -21,7 +22,14 @@ class MessageCreateRequest(BaseModel):
     chat_id: int
     type: MessageType = MessageType.TEXT
     text: str | None = Field(default=None, max_length=4096)
+    client_message_id: str | None = Field(default=None, min_length=8, max_length=64)
 
 
 class MessageUpdateRequest(BaseModel):
     text: str = Field(min_length=1, max_length=4096)
+
+
+class MessageStatusUpdateRequest(BaseModel):
+    chat_id: int
+    message_id: int
+    status: Literal["message_delivered", "message_read"]