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/tests/test_upload.py

@@ -558,8 +558,8 @@ def _display_write_sites() -> dict[tuple[str, str], set[str]]:
 # Bounded by `display_text` unless noted:
 #   service.redownload                CLEARS the field so the fresh metadata wins — `None` is the
 #                                     one value that needs no bound, and a clear is still a write.
-#   service.enqueue / enqueue_edit    `display_name` is already bounded by its caller; the fallback
-#                                     slices the YouTube title to the same width.
+#   service.enqueue / enqueue_edit /  `display_name` is already bounded by its caller; the fallback
+#   enqueue_clip                      slices the source's title to the same width.
 #   downloads.upload_download         passes through `spool_upload`'s value, bounded there.
 #
 # Held as EXACT EQUALITY rather than as a list of offenders. The offender form could only fail
@@ -573,6 +573,7 @@ DISPLAY_WRITE_SITES = {
         "if asset.status == 'ready' and asset.title else None)"
     },
     ("downloads/service.py", "enqueue_edit"): {"display_name or asset.title[:DISPLAY_TEXT_MAX]"},
+    ("downloads/service.py", "enqueue_clip"): {"display_name or asset.title[:DISPLAY_TEXT_MAX]"},
     ("downloads/upload.py", "ingest_upload"): {"display_text(display_name)"},
     ("routes/auth.py", "callback"): {"display_text(userinfo.get('name'))"},
     ("routes/auth.py", "_complete_link"): {"display_text(userinfo.get('name'))"},