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/app/api/v1/router.py

@@ -6,6 +6,7 @@ from app.api.v1.endpoints import (
     auth,
     budget,
     categories,
+    chat,
     exchange_rate,
     import_transactions,
     installments,
@@ -38,3 +39,4 @@ api_router.include_router(pensions.router)
 api_router.include_router(municipal_receipts.router)
 api_router.include_router(savings_accrual.router)
 api_router.include_router(sync_status.router)
+api_router.include_router(chat.router)