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/tests/test_download_output.py

@@ -144,7 +144,7 @@ class TestCodecsFromStreams:
 
 
 class TestRunFfprobeJson:
-    """The single ffprobe-JSON invocation point (shared by probe_codecs + upload._probe). A failed
+    """The single ffprobe-JSON invocation point (shared by probe_codecs + probe.probe_media). A failed
     probe or unparseable output must degrade to None, not raise — the malformed-input safety the old
     parse_probe_streams string parser used to own now lives here."""