feat(metadata): audio tag rewrite via ffmpeg + CBZ ComicInfo injection

Phases 5b and 5c of the metadata writer.

5b — single-file audio (MP3/FLAC/M4A/M4B): the container's tags are
rewritten with ffmpeg -c copy (no re-encode; -map 0 keeps cover art,
-map_chapters 0 keeps chapters). The key set is exactly what the
scanner's applyAudioTagFallbacks reads back — title+album, subtitle,
artist/album_artist, composer (narrators, Audible's convention),
grouping + iTunes series-part, date, genre — so a write + rescan
round-trips. Output is ffprobe-VALIDATED (duration within 1s/0.5% of
the source) before the atomic rename; a lossy container rewrite never
replaces a good book. Fields the format can't carry per our own read
chain are deliberately not written, and fields_written reports only
what really round-trips (subtitle rides MP3/FLAC — this image's mp4
muxer drops subtitle keys, discovered empirically and pinned by test).

5c — CBZ: ComicInfo.xml injected (or replaced) with the same atomic zip
rewrite as the EPUB path, extended with an append-if-missing mode
since a comic may not carry one yet. Field map mirrors the scanner's
comic block: Title/Series/Number/Count/Volume, Writer (authors),
Summary, Publisher, Year/Month/Day, LanguageISO, Web, Genre+Tags;
fractional series numbers are omitted rather than truncated to a lie.
CBR/CB7 stay on the sidecar path (RAR/7z can't be rewritten in place).

Integration tests generate media at test runtime with ffmpeg (m4b) and
in-memory zips (cbz) — no binary fixtures — pinning the tag round-trip
through ffprobe, post-write rescan titles, the ComicInfo injection and
the validation-gate behavior.

web/src/book-detail.ts

@@ -888,30 +888,41 @@ Alpine.data("bookDetail", () => {
     },
 
     // metadataWriteTarget mirrors services.MetadataWriteTargetFor: EPUB
-    // embeds its OPF, everything else takes a sidecar until the audio
-    // (5b) and CBZ (5c) writers land.
+    // embeds its OPF, single-file audio (mp3/flac/m4a/m4b) rewrites its
+    // container tags, CBZ gets ComicInfo.xml injected, and everything
+    // else takes a sidecar.
     metadataWriteTarget(): string {
       const ext = (this.fileWritePath.split(".").pop() || "").toLowerCase();
       if (ext === "epub" || ext === "kepub") return "embedded-opf";
+      if (["mp3", "flac", "m4a", "m4b"].includes(ext)) return "audio-tags";
+      if (ext === "cbz") return "comicinfo";
       return "sidecar-opf";
     },
 
     metadataWriteHint(): string {
-      if (this.metadataWriteTarget() === "embedded-opf") {
+      const target = this.metadataWriteTarget();
+      if (target === "embedded-opf") {
         return "Rewrite the metadata inside the EPUB, then rescan";
       }
+      if (target === "audio-tags") {
+        return "Rewrite the audiobook's embedded tags, then rescan";
+      }
+      if (target === "comicinfo") {
+        return "Write ComicInfo.xml inside the comic archive, then rescan";
+      }
       return "Write a Calibre-style metadata sidecar next to the file, then rescan";
     },
 
     async writeMetadataToFile() {
       if (this.writingFile) return;
-      if (
-        !window.confirm(
-          this.metadataWriteTarget() === "embedded-opf"
-            ? "Rewrite the metadata inside this EPUB with the current saved values? The original file will be modified (the SHA-256 changes, so device syncs re-key this book)."
-            : "Write a metadata sidecar next to this file with the current saved values? The media file itself is not modified.",
-        )
-      ) {
+      const target = this.metadataWriteTarget();
+      const confirmMsg =
+        target === "embedded-opf"
+          ? "Rewrite the metadata inside this EPUB with the current saved values? The original file will be modified (the SHA-256 changes, so device syncs re-key this book)."
+          : target === "audio-tags"
+            ? "Rewrite this audiobook's embedded tags with the current saved values? The audio streams are copied untouched, but the file's checksum changes, so device syncs re-key this book."
+            : "Write a metadata sidecar next to this file with the current saved values? The media file itself is not modified.";
+      if (!window.confirm(confirmMsg)) {
         return;
       }
       this.writingFile = true;
@@ -935,12 +946,15 @@ Alpine.data("bookDetail", () => {
             "error",
           );
         } else {
-          showToast(
+          const label =
             payload.metadata_target === "embedded-opf"
               ? "Metadata written into the EPUB and re-extracted"
-              : "Metadata sidecar written and re-extracted",
-            "success",
-          );
+              : payload.metadata_target === "audio-tags"
+                ? "Audio tags written and re-extracted"
+                : payload.metadata_target === "comicinfo"
+                  ? "ComicInfo.xml written and re-extracted"
+                  : "Metadata sidecar written and re-extracted";
+          showToast(label, "success");
         }
         setTimeout(() => window.location.reload(), 800);
       } catch (e) {