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/alembic/versions/e2fc4d65f93e_allow_deleted_messages_to_clear_content_.py

@@ -0,0 +1,39 @@
+"""allow deleted messages to clear content and attachments
+
+Revision ID: e2fc4d65f93e
+Revises: f3f255da9c96
+Create Date: 2026-08-28 16:20:02.114630
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'e2fc4d65f93e'
+down_revision: Union[str, Sequence[str], None] = 'f3f255da9c96'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.drop_constraint('messages_content_or_attachment_required', 'messages', type_='check')
+    op.create_check_constraint(
+        'messages_content_or_attachment_required',
+        'messages',
+        'content IS NOT NULL OR image_id IS NOT NULL OR file_id IS NOT NULL '
+        'OR deleted_at IS NOT NULL',
+    )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_constraint('messages_content_or_attachment_required', 'messages', type_='check')
+    op.create_check_constraint(
+        'messages_content_or_attachment_required',
+        'messages',
+        'content IS NOT NULL OR image_id IS NOT NULL OR file_id IS NOT NULL',
+    )