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/main.py

@@ -11,6 +11,7 @@ from jose import JWTError, jwt
 from pydantic import BaseModel
 
 from app.agent.agent import build_agent
+from app.agent.snapshot_store import PostgresSnapshotStore
 from app.agent.tools import reset_session, set_session
 from app.api.v1.router import api_router
 from app.auth import (
@@ -148,8 +149,17 @@ async def agent_auth_and_session(request: Request, call_next):
 # Register app routes
 app.include_router(api_router)
 
-# Mount the AG-UI agent endpoint.
-add_agent_framework_fastapi_endpoint(app, build_agent(), AGENT_PATH)
+# Mount the AG-UI agent endpoint. The snapshot store persists the chat
+# thread across page reloads; scope is a constant because this is a
+# single-user app and auth is already enforced by agent_auth_and_session
+# (the scope resolver only sees the AGUIRequest, which carries no identity).
+add_agent_framework_fastapi_endpoint(
+    app,
+    build_agent(),
+    AGENT_PATH,
+    snapshot_store=PostgresSnapshotStore(),
+    snapshot_scope_resolver=lambda _request: "default",
+)
 
 
 @app.get("/")