fix(downloads): a conversion's commit, a stopped upload, the fast join and a clip's copy hold up

- The fast join gives each ffconcat entry its own media-only format whitelist, so a library file
  can no longer be opened as a nested concat list (the list's whitelist was handed down to it).
- A file the guarded probe cannot read is a download-only file, not a guess from yt-dlp's info.
- A conversion's or an ingest's commit that raises is re-read in a fresh session before any file
  is removed: a commit that landed keeps the file its row now names.
- An original that cannot be removed after conversion is logged; the upload stays ready.
- An upload whose conversion is cancelled or deleted midway is freed once nothing holds it; a
  paused one is kept.
- A shared link says "preparing" only while its video's job is queued or running.
- The admin storage view counts every file on disk ("Files on disk"), not only ready ones.
- A clip's failure title, unknown failure and cancel dialog, and a clip's and an upload's delete
  dialog speak of what the job made; an upload's quota refusal explains that the original is kept.
- /watch announces "preparing" as an alert and moves focus to the new Retry after an action.

frontend/src/lib/api/downloads.ts

@@ -47,7 +47,7 @@ export type DownloadStatus = "queued" | "running" | "paused" | "done" | "error"
 // "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.
-type DownloadJobKind = "download" | "edit" | "upload";
+export type DownloadJobKind = "download" | "edit" | "upload";
 
 // 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).
@@ -137,7 +137,7 @@ export interface DownloadUsage {
 }
 
 export interface AdminStorage {
-  ready_files: number;
+  files_on_disk: number;
   total_bytes: number;
   total_assets: number;
   total_cap_bytes: number;