fix(06): WR-07 replace print() with structlog for cloud delete errors

Replace print(..., file=sys.stderr) with structlog.get_logger(__name__)
.warning("cloud_delete_failed", ...) so cloud delete errors carry a
correlation ID, are picked up by promtail, and appear in Loki — matching
the Phase 6 goal of routing all logging through structlog.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/documents.py

@@ -26,6 +26,9 @@ import uuid
 from pathlib import Path
 from typing import Optional
 
+import structlog as _structlog
+_log = _structlog.get_logger(__name__)
+
 from fastapi import APIRouter, Depends, Form, HTTPException, Query, Request, UploadFile, File, status
 from fastapi.responses import JSONResponse, StreamingResponse
 from pydantic import BaseModel, Field, field_validator
@@ -668,8 +671,7 @@ async def delete_document(
             cloud_backend = await get_storage_backend_for_document(doc, current_user, session)
             await cloud_backend.delete_object(doc.object_key)
         except Exception as exc:
-            import sys
-            print(f"[cloud-delete] provider error: {exc}", file=sys.stderr)
+            _log.warning("cloud_delete_failed", provider=doc.storage_backend, error=str(exc))
             return JSONResponse(
                 status_code=200,
                 content={