feat(downloads): every ready file is phone-safe — uploads and clips too

A share link is opened on whatever the friend holds. Measured on an iPhone and an iPad: AV1,
Matroska, HEVC tagged hev1 and MP3 inside an .mp4 fail there; Chrome and Firefox add HEVC and
10-bit / 4:4:4 H.264. The old check judged the codec NAME only and ran on downloads only, so a
10-bit H.264 passed, an upload went out exactly as it came (an iPhone's HEVC .mov, a webm, an mkv)
and a fast trim kept its source's codecs.

One rule now, `downloads/playable.py`: video is H.264 8-bit 4:2:0 + AAC in .mp4, audio-only is
.mp3 or AAC .m4a; `plan` says what must change, `build_cmd` re-encodes only the failing stream (or
remuxes). The worker applies it to a download, an editor clip and — new — an upload: media that
fails the rule is queued (asset pending, job queued) and the worker converts it, keeping only the
converted file (owner's choice). Until then the original sits at rel_path, so Retry starts from it
and a delete removes it; an upload that lost its last holder mid-conversion is reaped by the GC.
Every libx264 run (edit, concat, conversion) now pins `-pix_fmt yuv420p` — without it a 10-bit
source came out High 10. The download cache signature moves to v5 so a cached 10-bit asset
re-derives. The upload toast says when the file is still converting (HU/EN).

Tests: the plan for every measured class; real ffmpeg turning a 10-bit Matroska + Opus and an
mp3-in-mp4 into files the rule accepts (red without the pixel-format pin); the queued upload's
delete, cancel-then-GC, failed conversion and real worker conversion (the cleanup tests red
without their rules); the toast (red without the branch). Live on localdev: 9 fixture uploads
converted, every served file probed H.264 High yuv420p + AAC, or mp3 / m4a.

frontend/src/i18n/locales/en/downloads.json

@@ -36,6 +36,7 @@
     "uploading": "Uploading… {{pct}}%",
     "uploadProgress": "Upload progress",
     "added": "File uploaded",
+    "addedConverting": "File uploaded — converting it so it plays on any phone",
     "failed": "Couldn't upload the file"
   },
   "tabs": {

frontend/src/i18n/locales/hu/downloads.json

@@ -36,6 +36,7 @@
     "uploading": "Feltöltés… {{pct}}%",
     "uploadProgress": "Feltöltés folyamata",
     "added": "Fájl feltöltve",
+    "addedConverting": "Fájl feltöltve — átalakítjuk, hogy minden telefonon lejátszható legyen",
     "failed": "Nem sikerült feltölteni a fájlt"
   },
   "tabs": {