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/lib/api/downloads.ts

@@ -196,7 +196,8 @@ export const downloadsApi = {
   }): Promise<DownloadJob> => req("/api/downloads", { method: "POST", body: JSON.stringify(body) }),
   // Ingest a local file into the library. Sent via XHR rather than `req` so the dialog can
   // show upload progress; any file type is accepted (media gets the full function set, a generic
-  // file is download-only). Returns the already-`done` job the ingest creates.
+  // file is download-only). Returns the job the ingest creates: `done`, or `queued` while the
+  // worker converts media no phone could play as it is (`downloads/upload.py`).
   // The body IS the file, and the two names ride as headers. Not a style choice: a multipart part is
   // a DECLARED body, which FastAPI reads and spools BEFORE the route's `current_user` dependency
   // runs — so with the old shape an unauthenticated caller could make the server write the whole