Apply minified serialization to books inside collections

The GET /api/libraries/:id/collections endpoint captured the minified
flag on the response payload but never passed it down to the book
serialization, so each book was always returned fully expanded with its
complete libraryFiles array and heavy media fields (audioFiles, chapters,
tracks). For libraries with multi-file audiobooks this produced responses
of tens of megabytes even when minified=1 was requested.

Thread the minified flag through getOldCollectionsJsonExpanded and
Collection.toOldJSONExpanded so that, when requested, books use the
existing library item minified form. Non-minified requests are unchanged.

Fixes #5170

test/server/models/Collection.test.js

@@ -0,0 +1,107 @@
+const { expect } = require('chai')
+const { Sequelize } = require('sequelize')
+
+const Database = require('../../../server/Database')
+
+describe('Collection', () => {
+  /** @type {string} */
+  let libraryId
+  /** @type {string} */
+  let collectionId
+
+  beforeEach(async () => {
+    global.ServerSettings = {}
+    Database.sequelize = new Sequelize({ dialect: 'sqlite', storage: ':memory:', logging: false })
+    Database.sequelize.uppercaseFirst = (str) => (str ? `${str[0].toUpperCase()}${str.substr(1)}` : '')
+    await Database.buildModels()
+
+    const library = await Database.libraryModel.create({ name: 'Test Library', mediaType: 'book' })
+    libraryId = library.id
+    const libraryFolder = await Database.libraryFolderModel.create({ path: '/test', libraryId: library.id })
+
+    // Build a book with heavy media: many audio files + chapters
+    const audioFiles = []
+    const chapters = []
+    for (let i = 0; i < 30; i++) {
+      audioFiles.push({
+        index: i + 1,
+        ino: `ino-${i}`,
+        duration: 600,
+        exclude: false,
+        mimeType: 'audio/mpeg',
+        metadata: { filename: `track-${i}.mp3`, ext: '.mp3', path: `/test/book/track-${i}.mp3`, relPath: `track-${i}.mp3`, size: 12345678 }
+      })
+      chapters.push({ id: i, start: i * 600, end: (i + 1) * 600, title: `Chapter ${i + 1}` })
+    }
+
+    const book = await Database.bookModel.create({ title: 'Heavy Book', audioFiles, tags: [], narrators: [], genres: [], chapters })
+
+    // libraryItem carries a large libraryFiles array (dropped by the minified form)
+    const libraryFiles = []
+    for (let i = 0; i < 30; i++) {
+      libraryFiles.push({
+        ino: `lf-${i}`,
+        metadata: { filename: `track-${i}.mp3`, ext: '.mp3', path: `/test/book/track-${i}.mp3`, relPath: `track-${i}.mp3`, size: 12345678, mtimeMs: 0, ctimeMs: 0, birthtimeMs: 0 }
+      })
+    }
+    await Database.libraryItemModel.create({ libraryFiles, mediaId: book.id, mediaType: 'book', libraryId: library.id, libraryFolderId: libraryFolder.id })
+
+    const collection = await Database.collectionModel.create({ name: 'Test Collection', libraryId: library.id })
+    collectionId = collection.id
+    await Database.collectionBookModel.create({ collectionId: collection.id, bookId: book.id, order: 1 })
+  })
+
+  afterEach(async () => {
+    await Database.sequelize.close()
+  })
+
+  describe('getOldCollectionsJsonExpanded', () => {
+    it('returns fully expanded book objects when minified is not requested', async () => {
+      const collections = await Database.collectionModel.getOldCollectionsJsonExpanded(null, libraryId, [], false)
+      expect(collections).to.have.lengthOf(1)
+      const book = collections[0].books[0]
+
+      // Library item keeps the full libraryFiles array
+      expect(book).to.have.property('libraryFiles')
+      expect(book.libraryFiles).to.be.an('array').with.lengthOf(30)
+
+      // Media keeps the heavy arrays
+      expect(book.media).to.have.property('audioFiles').that.is.an('array').with.lengthOf(30)
+      expect(book.media).to.have.property('chapters').that.is.an('array').with.lengthOf(30)
+      expect(book.media).to.have.property('tracks').that.is.an('array').with.lengthOf(30)
+    })
+
+    it('returns minified book objects that omit libraryFiles and heavy media when minified is requested', async () => {
+      const collections = await Database.collectionModel.getOldCollectionsJsonExpanded(null, libraryId, [], true)
+      expect(collections).to.have.lengthOf(1)
+      const book = collections[0].books[0]
+
+      // Library item drops libraryFiles in favor of a numFiles count
+      expect(book).to.not.have.property('libraryFiles')
+      expect(book).to.have.property('numFiles', 30)
+
+      // Media drops the heavy arrays in favor of counts
+      expect(book.media).to.not.have.property('audioFiles')
+      expect(book.media).to.not.have.property('chapters')
+      expect(book.media).to.not.have.property('tracks')
+      expect(book.media).to.have.property('numAudioFiles', 30)
+      expect(book.media).to.have.property('numChapters', 30)
+      expect(book.media).to.have.property('numTracks', 30)
+
+      // Essential display fields survive minification
+      expect(book.media).to.have.property('coverPath')
+      expect(book.media).to.have.property('duration')
+      expect(book.media.metadata).to.have.property('title', 'Heavy Book')
+    })
+
+    it('produces a materially smaller payload when minified', async () => {
+      const expanded = await Database.collectionModel.getOldCollectionsJsonExpanded(null, libraryId, [], false)
+      const minified = await Database.collectionModel.getOldCollectionsJsonExpanded(null, libraryId, [], true)
+
+      const expandedSize = JSON.stringify(expanded).length
+      const minifiedSize = JSON.stringify(minified).length
+
+      expect(minifiedSize).to.be.lessThan(expandedSize * 0.5)
+    })
+  })
+})