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/endpoints/chat.py

@@ -0,0 +1,41 @@
+"""Assistant chat thread management.
+
+The conversation itself is persisted by the AG-UI snapshot store
+(app/agent/snapshot_store.py) under a single ("default", "main") thread;
+this router only exposes the reset used by "Nueva conversación".
+"""
+
+from fastapi import APIRouter, Depends
+from pydantic import BaseModel
+from sqlmodel import Session, select
+
+from app.auth import get_current_user
+from app.db import get_session
+from app.models.models import ChatThreadSnapshot
+
+router = APIRouter(prefix="/chat", tags=["chat"])
+
+CHAT_SCOPE = "default"
+CHAT_THREAD_ID = "main"
+
+
+class ClearThreadResponse(BaseModel):
+    cleared: bool
+
+
+@router.delete("/thread", response_model=ClearThreadResponse)
+def clear_thread(
+    session: Session = Depends(get_session),
+    _user: str = Depends(get_current_user),
+):
+    row = session.exec(
+        select(ChatThreadSnapshot).where(
+            ChatThreadSnapshot.scope == CHAT_SCOPE,
+            ChatThreadSnapshot.thread_id == CHAT_THREAD_ID,
+        )
+    ).first()
+    if row is None:
+        return {"cleared": False}
+    session.delete(row)
+    session.commit()
+    return {"cleared": True}

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)