feat(downloads): ASCII-slug names, universal browser playability, force re-download

Mini-epic "Downloads correctness & UX" (S1–S3), on top of the iOS-AV1 + 720p fixes.

S1 — naming & titles:
- ASCII-fold on-disk dir/file names AND the served (Content-Disposition) filename: no
  spaces (underscore-joined), no accents (á→a, ő→o, ß→ss), pure ASCII — portable and free
  of the `?` mojibake accented names show in non-UTF-8 tools. (Forward-only; existing files
  clean up via re-download.)
- Strip a leading social engagement prefix ("1.6M views · 66K reactions | …") from titles.

S2 — universal browser playability (generalizes the iOS re-encode):
- ensure_browser_playable guarantees the stored file is H.264 + AAC/MP3 in mp4 — the only
  combo every browser decodes. Re-encode just the offending stream(s) (video for AV1/VP9/HEVC,
  audio for Opus/etc), or remux when only the container is wrong. Gated on the mp4 compat
  profile (an explicit vcodec / non-mp4 custom profile opts out).

S3 — force re-download (preserves share links):
- POST /downloads/{job_id}/redownload keeps the job row so its public DownloadLink survives
  and resolves to the freshly-downloaded file; deletes the old file and re-fetches under the
  current rules. A failed re-download leaves the job in error (the intended broken-link case).
- Library "Re-download" action (icon + confirm), api method, hu/en strings.

Tests: naming/title-strip (test_naming), browser_transcode_flags matrix; existing
download_filename expectation updated for the underscore policy.

frontend/src/components/DownloadCenter.tsx

@@ -9,6 +9,7 @@ import {
   Pause,
   Play,
   Plus,
+  RotateCcw,
   Scissors,
   Settings2,
   Share2,
@@ -672,14 +673,22 @@ export default function DownloadCenter() {
 
   const invalidate = () => invalidateDownloads(qc);
   const act = useMutation({
-    mutationFn: ({ id, op }: { id: number; op: "pause" | "resume" | "cancel" | "delete" }) =>
+    mutationFn: ({
+      id,
+      op,
+    }: {
+      id: number;
+      op: "pause" | "resume" | "cancel" | "delete" | "redownload";
+    }) =>
       op === "pause"
         ? api.pauseDownload(id)
         : op === "resume"
           ? api.resumeDownload(id)
           : op === "cancel"
             ? api.cancelDownload(id)
-            : api.deleteDownload(id),
+            : op === "redownload"
+              ? api.redownloadDownload(id)
+              : api.deleteDownload(id),
     onSuccess: invalidate,
     // No onError: a failed job action returns 400/409/422/500, already surfaced by the global
     // error modal (api.ts req) with the backend's specific message — a caller toast would double it.
@@ -824,6 +833,18 @@ export default function DownloadCenter() {
                   <IconBtn onClick={() => setShareJob(job)} title={t("downloads.actions.share")}>
                     <Share2 className="w-4 h-4" />
                   </IconBtn>
+                  <IconBtn
+                    onClick={() =>
+                      confirmThen(
+                        t("downloads.confirm.redownloadTitle"),
+                        t("downloads.confirm.redownloadBody"),
+                        () => act.mutate({ id: job.id, op: "redownload" }),
+                      )
+                    }
+                    title={t("downloads.actions.redownload")}
+                  >
+                    <RotateCcw className="w-4 h-4" />
+                  </IconBtn>
                   <IconBtn
                     onClick={() =>
                       confirmThen(

frontend/src/i18n/locales/en/downloads.json

@@ -59,6 +59,7 @@
     "editDetails": "Edit details",
     "share": "Share",
     "edit": "Edit / trim",
+    "redownload": "Re-download (rebuild file)",
     "removeShared": "Remove from my list"
   },
   "empty": {
@@ -125,7 +126,9 @@
     "deleteTitle": "Remove download?",
     "deleteBody": "This removes it from your list. The file may stay cached for other users until it expires.",
     "removeSharedTitle": "Remove from your list?",
-    "removeSharedBody": "This removes it from your shared list only — it won't delete the owner's file."
+    "removeSharedBody": "This removes it from your shared list only — it won't delete the owner's file.",
+    "redownloadTitle": "Re-download this file?",
+    "redownloadBody": "The current file is deleted and downloaded again under the latest format and naming rules. Any share links you've sent keep working and will point to the new file (a failed re-download leaves the link broken)."
   },
   "profiles": {
     "manage": "Manage formats",

frontend/src/i18n/locales/hu/downloads.json

@@ -59,6 +59,7 @@
     "editDetails": "Adatok szerkesztése",
     "share": "Megosztás",
     "edit": "Szerkesztés / vágás",
+    "redownload": "Újraletöltés (fájl újraépítése)",
     "removeShared": "Eltávolítás a listámból"
   },
   "empty": {
@@ -125,7 +126,9 @@
     "deleteTitle": "Eltávolítod a letöltést?",
     "deleteBody": "Ez eltávolítja a listádról. A fájl a lejáratáig még a gyorsítótárban maradhat mások számára.",
     "removeSharedTitle": "Eltávolítod a listádból?",
-    "removeSharedBody": "Csak a te megosztott listádból tűnik el — a tulajdonos fájlját nem törli."
+    "removeSharedBody": "Csak a te megosztott listádból tűnik el — a tulajdonos fájlját nem törli.",
+    "redownloadTitle": "Újratöltöd ezt a fájlt?",
+    "redownloadBody": "A jelenlegi fájl törlődik és újra letöltődik a legfrissebb formátum- és elnevezési szabályokkal. A már elküldött megosztási linkek működnek tovább, és az új fájlra fognak mutatni (sikertelen újratöltés esetén a link törötten marad)."
   },
   "profiles": {
     "manage": "Formátumok kezelése",

frontend/src/lib/api.ts

@@ -1673,6 +1673,10 @@ export const api = {
     req(`/api/downloads/${id}/resume`, { method: "POST" }, { idempotent: true }),
   cancelDownload: (id: number): Promise<DownloadJob> =>
     req(`/api/downloads/${id}/cancel`, { method: "POST" }, { idempotent: true }),
+  // Force a fresh re-download of a finished/failed job — keeps the job (and its share links),
+  // deletes the old file, and rebuilds it under the current format/naming rules.
+  redownloadDownload: (id: number): Promise<DownloadJob> =>
+    req(`/api/downloads/${id}/redownload`, { method: "POST" }, { idempotent: true }),
   deleteDownload: (id: number) => req(`/api/downloads/${id}`, { method: "DELETE" }),
   // Update a download's display metadata (title / channel name+link / extra reference URLs).
   updateDownloadMeta: (