feat(books): filter read-along titles

Expose read-along EPUBs as an additive format filter while preserving ordinary EPUB matching and existing client compatibility.

Closes #1583

packages/types/src/app-info.ts

@@ -39,6 +39,8 @@ export const SERVER_FEATURES = [
   "metadata-search-results-per-provider",
   /** Book and custom date filters accept the `moreThanAgo` rolling-boundary operator. */
   "relative-date-ranges",
+  /** The `format` book filter accepts `epub:readalong` for EPUBs with media overlays. */
+  "read-along-filter",
 ] as const;
 
 export type ServerFeature = (typeof SERVER_FEATURES)[number];

packages/types/src/book.ts

@@ -8,12 +8,17 @@ import type { CoverAspectRatio } from "./library";
 import { DEFAULT_FORMAT_PRIORITY } from "./library";
 import type { SeriesIndex } from "./series-index";
 import type { EpubMediaOverlayCapability } from "./epub";
+import { READ_ALONG_FORMAT_PRIORITY } from "./format-priority";
 
 // Derived rather than duplicated: these two lists describe the same set of formats,
 // and maintaining them separately let BOOK_FORMATS fall behind on azw and kepub.
 export const BOOK_FORMATS = DEFAULT_FORMAT_PRIORITY;
 export type BookFormat = (typeof BOOK_FORMATS)[number];
 
+/** Values accepted by the format filter. Read-along is an edition key, not a physical file format. */
+export const BOOK_FILTER_FORMATS = [...BOOK_FORMATS, READ_ALONG_FORMAT_PRIORITY] as const;
+export type BookFilterFormat = (typeof BOOK_FILTER_FORMATS)[number];
+
 /** Exported as an ordered list too, so a form offering these cannot drift from what matches them. */
 export const AUDIO_FORMAT_LIST = ["m4b", "mp3", "m4a", "opus", "ogg", "flac"] as const;
 const AUDIO_FORMATS = new Set<string>(AUDIO_FORMAT_LIST);
@@ -440,7 +445,7 @@ export type BooksPage = {
 };
 
 export type BookFormatFacet = {
-  format: BookFormat;
+  format: BookFilterFormat;
   count: number;
 };