feat(downloads): ad-hoc clips cut straight from a YouTube video or a Plex item

POST /api/downloads/clip takes a source (a YouTube id or a Plex rating key), a range of at most
ten minutes and, for Plex, the audio track being heard. The clip becomes a per-user asset in the
library like an editor clip, shareable as soon as it is ready. A YouTube clip fetches only its
range (yt-dlp download_ranges: 3-7 s whatever the video's length); a Plex clip reads the library
file in place. Both then go through one re-encode: frame-exact at the marks, H.264 at most 1080p
with HDR tonemapped to SDR, stereo AAC, no chapter track, .mp4.

Job kinds are a JobKind StrEnum now, and Re-download refuses every kind but a download.

frontend/src/components/downloadRow.tsx

@@ -268,7 +268,7 @@ export function DownloadRow({
           <div
             className={`font-medium leading-snug break-words line-clamp-1 @max-[40rem]/dlrow:line-clamp-2`}
           >
-            {job.job_kind === "edit" && (
+            {(job.job_kind === "edit" || job.job_kind === "clip") && (
               <span className="mr-1.5 inline-flex items-center gap-1 px-1.5 py-0.5 align-middle rounded-sm text-2xs font-semibold uppercase tracking-wide bg-accent/15 text-accent">
                 <Scissors className="w-3 h-3" /> {t("downloads.clipBadge")}
               </span>

frontend/src/components/useDownloadActions.tsx

@@ -59,6 +59,7 @@ const CANCEL_COPY: Record<DownloadJobKind, typeof CANCEL_DOWNLOAD> = {
   download: CANCEL_DOWNLOAD,
   upload: CANCEL_UPLOAD,
   edit: CANCEL_EDIT,
+  clip: CANCEL_EDIT, // both are a clip being cut: the same words
 };
 // Deleting a download may leave its shared cache file for other holders. An upload and a clip are
 // their owner's alone (a clip's asset is keyed by its user), so deleting one removes the file for good.
@@ -69,6 +70,7 @@ const DELETE_COPY: Record<DownloadJobKind, { title: string; body: string }> = {
     body: "downloads.confirm.deleteUploadBody",
   },
   edit: { title: "downloads.confirm.deleteEditTitle", body: "downloads.confirm.deleteEditBody" },
+  clip: { title: "downloads.confirm.deleteEditTitle", body: "downloads.confirm.deleteEditBody" },
 };
 
 export function useDownloadActions({