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.

backend/app/downloads/quota.py

@@ -24,7 +24,7 @@ from sqlalchemy import and_, func, or_, select
 from sqlalchemy.orm import Session
 
 from app import sysconfig
-from app.downloads.status import JobStatus
+from app.downloads.status import JobKind, JobStatus
 from app.models import DownloadJob, DownloadQuota, MediaAsset
 
 # Jobs a user is considered to be "holding" (occupy a job slot / footprint). Terminal failures
@@ -86,7 +86,7 @@ def footprint(db: Session, user_id: int) -> int:
             DownloadJob.user_id == user_id,
             or_(
                 DownloadJob.status.in_(_HOLDING),
-                and_(DownloadJob.job_kind == "upload", DownloadJob.status == JobStatus.ERROR),
+                and_(DownloadJob.job_kind == JobKind.UPLOAD, DownloadJob.status == JobStatus.ERROR),
             ),
             DownloadJob.asset_id.is_not(None),
         )