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/models/message.py

@@ -10,8 +10,12 @@ from app.models.base import Base
 class Message(Base):
     __tablename__ = "messages"
     __table_args__ = (
+        # #53: a deleted message clears content/image_id/file_id entirely
+        # (see message_service.delete_message) -- the "must have something"
+        # rule only applies while the message is actually live.
         CheckConstraint(
-            "content IS NOT NULL OR image_id IS NOT NULL OR file_id IS NOT NULL",
+            "content IS NOT NULL OR image_id IS NOT NULL OR file_id IS NOT NULL "
+            "OR deleted_at IS NOT NULL",
             name="messages_content_or_attachment_required",
         ),
     )