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/components/WatchPage.dom.test.tsx

@@ -224,13 +224,33 @@ describe("WatchPage honest failures", () => {
     );
     const page = render(<WatchPage />);
     const retry = await page.findByRole("button", { name: "Try again" });
-    expect(page.getByRole("status").textContent).toContain("This video is being prepared.");
+    expect(page.getByRole("alert").textContent).toContain("This video is being prepared.");
     expect(page.queryByText(GONE)).toBeNull();
 
     fireEvent.click(retry);
     await waitFor(() => expect(page.container.querySelector("video")).not.toBeNull());
   });
 
+  it("a Retry that is answered 'still preparing' says so and keeps the focus on Retry", async () => {
+    // The answer replaces the button the viewer was on: announced (an alert, inserted filled), and
+    // the new Retry takes the focus instead of leaving it on <body>.
+    window.history.replaceState({}, "", "/watch/tok-preparing");
+    const preparing = () => res(404, { detail: { code: 404, reason: "video_preparing" } });
+    vi.stubGlobal(
+      "fetch",
+      vi.fn().mockImplementation(() => Promise.resolve(preparing())),
+    );
+    const page = render(<WatchPage />);
+    const first = await page.findByRole("button", { name: "Try again" });
+    expect(document.activeElement).not.toBe(first); // the first load moves no focus
+
+    fireEvent.click(first);
+    await waitFor(() =>
+      expect(document.activeElement).toBe(page.getByRole("button", { name: "Try again" })),
+    );
+    expect(page.getByRole("alert").textContent).toContain("This video is being prepared.");
+  });
+
   async function tryPassword(second: () => Promise<unknown>) {
     vi.stubGlobal(
       "fetch",