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/lib/api/downloads.ts

@@ -44,10 +44,10 @@ interface DownloadAsset {
 }
 
 export type DownloadStatus = "queued" | "running" | "paused" | "done" | "error" | "canceled";
-// "download" (default), "edit" (a per-user trim/crop derivative cut from another job), or "upload"
-// (a locally-uploaded file — no re-fetchable source, so re-download is rejected backend-side).
-// Local (not exported): consumers compare `job.job_kind` against literals, never name the type.
-export type DownloadJobKind = "download" | "edit" | "upload";
+// "download" (default), "edit" (a per-user trim/crop derivative cut from another job), "upload"
+// (a locally-uploaded file) or "clip" (a marked range of a YouTube video or a Plex item, cut straight
+// from the source). Only a download has a re-fetchable source; re-download refuses the rest.
+export type DownloadJobKind = "download" | "edit" | "upload" | "clip";
 
 // Phase-2 editor recipe. A single `trim` (one output file) or a `segments` cut-list joined into
 // one file. `accurate` re-encodes for a frame-accurate cut (a crop always re-encodes).