refactor: address /code-review — rename uploader_url() to build_uploader_url()

The R12 S3 underscore-publish left worker.uploader_url() sharing a name with the
`uploader_url` model attribute and the yt-dlp `"uploader_url"` info-dict key. The
function is now build_uploader_url() — the attribute, dict key, log line and
docstrings keep the plain `uploader_url` name. Rename touches only the def, its
two call sites, and the backfill import; no behaviour change (ruff clean,
pytest 193).

backend/app/downloads/backfill.py

@@ -21,7 +21,7 @@ from app.db import SessionLocal
 from app.downloads import edit as editmod
 from app.downloads import formats, service
 from app.models import MediaAsset
-from app.worker import uploader_url
+from app.worker import build_uploader_url
 
 log = logging.getLogger("siftlode.backfill")
 
@@ -73,7 +73,7 @@ def run() -> dict:
             if a.uploader_url is None and _should_refetch(a):
                 url = a.source_webpage_url or service.source_url(a.source_kind, a.source_ref)
                 info = _extract_meta(url)
-                uu = uploader_url(info) if info else None
+                uu = build_uploader_url(info) if info else None
                 if uu:
                     a.uploader_url = uu
                     filled_url += 1