Assistant: persist the chat thread in Postgres

Back MAF's opt-in AG-UI snapshot persistence with a DB store so the
conversation survives page reloads and restarts:

- ChatThreadSnapshot: one upserted row per (scope, thread_id), JSONB
  messages/state/interrupt (migration 794baf50635b)
- PostgresSnapshotStore implements the AGUIThreadSnapshotStore protocol.
  It opens its own sessions (MAF saves during SSE streaming, after the
  request middleware has closed the ContextVar session) and sanitizes on
  save: camelCase toolCalls/toolCallId (the @ag-ui/client Zod schema
  strips snake_case, which would lose cards on replay), drop BFF ctx-*
  context messages, dedupe MAF's re-recorded multi-run turns, and drop
  orphaned tool results
- Endpoint mount gains snapshot_store + a constant scope resolver
  (single-user app; auth already enforced by agent_auth_and_session)
- DELETE /api/v1/chat/thread resets the conversation

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

backend/alembic/versions/794baf50635b_chat_thread_snapshot.py

@@ -0,0 +1,45 @@
+"""chat thread snapshot
+
+Revision ID: 794baf50635b
+Revises: 961802a2f50d
+Create Date: 2026-07-04 17:49:14.914103
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+import sqlmodel
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = '794baf50635b'
+down_revision: Union[str, Sequence[str], None] = '961802a2f50d'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('chatthreadsnapshot',
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('scope', sqlmodel.sql.sqltypes.AutoString(), nullable=False),
+    sa.Column('thread_id', sqlmodel.sql.sqltypes.AutoString(), nullable=False),
+    sa.Column('messages', sa.JSON().with_variant(postgresql.JSONB(astext_type=sa.Text()), 'postgresql'), server_default='[]', nullable=False),
+    sa.Column('state', sa.JSON().with_variant(postgresql.JSONB(astext_type=sa.Text()), 'postgresql'), nullable=True),
+    sa.Column('interrupt', sa.JSON().with_variant(postgresql.JSONB(astext_type=sa.Text()), 'postgresql'), nullable=True),
+    sa.Column('updated_at', sa.DateTime(), nullable=False),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('scope', 'thread_id')
+    )
+    op.create_index(op.f('ix_chatthreadsnapshot_scope'), 'chatthreadsnapshot', ['scope'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_chatthreadsnapshot_scope'), table_name='chatthreadsnapshot')
+    op.drop_table('chatthreadsnapshot')
+    # ### end Alembic commands ###