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_phone_safe_contract.py

@@ -73,10 +73,17 @@ def test_every_path_to_ready_runs_the_rule():
             found.append(where)
             if not runs_rule:
                 skipped.append(where)
-    # The paths known today: a download, an editor clip, an upload's conversion, an upload kept as is.
+    # The paths known today: a download, an editor clip, an ad-hoc clip, an upload's conversion, an
+    # upload kept as is.
     # Fewer means the scan went blind (a rename, a new status spelling) — not that a path is gone.
     names = {w.rsplit(" ", 1)[1] for w in found}
-    assert {"_download", "_process_edit", "_convert_upload", "ingest_upload"} <= names, found
+    assert {
+        "_download",
+        "_process_edit",
+        "_process_clip",
+        "_convert_upload",
+        "ingest_upload",
+    } <= names, found
     assert skipped == [], f"marks an asset ready without the phone-safe rule: {skipped}"