Add the ability to delete a message (#53)

Message.deleted_at has existed since the initial schema but was never
wired up -- no WS envelope, no permission check, no frontend concept of
it at all. Soft delete, author-only (mirrors the existing edit
permission exactly): content and any attached image/file are cleared
and the underlying MessageImage/MessageFile row and stored file are
actually removed, not just detached, so the message becomes a "This
message was deleted" tombstone with nothing left to recover through a
stale attachment URL. A deleted message can no longer be edited or
reacted to.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas/message.py

@@ -44,3 +44,9 @@ class MessageRead(BaseModel):
     reactions: list[ReactionSummary]
     created_at: datetime
     edited_at: datetime | None
+    # #53: null for a live message; set once deleted, at which point
+    # content/image_id/file/link_preview are all already cleared
+    # server-side (see message_service.delete_message). `reactions` isn't
+    # cleared server-side -- the frontend just doesn't render them once
+    # deleted_at is set, same as it doesn't render the rest of a tombstone.
+    deleted_at: datetime | None