fix(downloads): classify media_kind from a file probe, not yt-dlp info

A downloaded video was silently demoted to a download-only "file" (no
in-app play/edit) whenever yt-dlp's info dict omitted vcodec/acodec — which
it does for Facebook and other sources. Finalize derived media_kind from
those nulls (`derive(None, None)` -> file), so a genuine H.264/AAC MP4
became non-playable in the library.

Fix: probe the finished file on disk (the ground truth) at finalize and
derive from that, falling back to the info dict only for a field the probe
couldn't read. This is the same ffprobe the upload ingest already used, now
extracted to a shared downloads/probe.py so the download worker and the
upload path share one classifier (and can't drift). Also backfills the
stored vcodec/acodec/width/height from the real file.

Tests: TestResolveMediaFacts pins probe-wins-over-info (incl. the Facebook
no-codec case and yt-dlp's literal "none"); upload's probe tests repoint to
the shared module.

backend/app/downloads/edit.py

@@ -447,9 +447,9 @@ def run_ffprobe_json(
 ) -> dict | None:
     """One ffprobe call → its parsed JSON dict, or None on any failure (timeout, non-zero exit,
     missing binary, unparseable output). The SINGLE ffprobe-JSON-invocation point in the downloads
-    package: the browser-playability codec probe here and the upload-ingest probe (`upload._probe`)
-    both go through it, so the subprocess flags/timeout/error handling live in one place instead of
-    two drifting copies."""
+    package: the browser-playability codec probe here and the shared media probe (`probe.probe_media`,
+    used by both the download worker's finalize and the upload ingest) both go through it, so the
+    subprocess flags/timeout/error handling live in one place instead of drifting copies."""
     args = ["ffprobe", "-v", "error"]
     if show_streams:
         args.append("-show_streams")