fix(opds): send book descriptions as plain text

Descriptions are stored as HTML but the feed sent them escaped under
type="text", so clients that honour the type, such as Foliate, showed the
tags literally. Flatten them with htmlToPlainText, keeping line breaks.

Fixes #1569

server/src/modules/opds/__tests__/opds.service.test.ts

@@ -522,4 +522,40 @@ describe('OpdsService', () => {
       expect(xml).toContain(''');
     });
   });
+
+  describe('book descriptions', () => {
+    function feedFor(description: string | null): string {
+      return makeService().generateAcquisitionFeed(
+        'Catalog',
+        'urn:bookorbit:catalog',
+        [sampleBook({ description })],
+        1,
+        1,
+        50,
+        `${BASE}/catalog?page=1&size=50`,
+        'test-token',
+      );
+    }
+
+    it('sends an HTML description as plain text with its paragraph and line breaks', () => {
+      const xml = feedFor('<p>The <strong>first</strong> book &amp; its sequel.</p><p>Line one<br>line two</p>');
+
+      expect(xml).toContain('<content type="text">The first book &amp; its sequel.\nLine one\nline two</content>');
+      expect(xml).not.toContain('&lt;p&gt;');
+      expect(xml).not.toContain('&lt;strong&gt;');
+    });
+
+    it('keeps a plain-text description and its line breaks unchanged', () => {
+      const xml = feedFor('Part one.\n\nPart two.');
+
+      expect(xml).toContain('<content type="text">Part one.\n\nPart two.</content>');
+    });
+
+    it.each([
+      ['no description', null],
+      ['a description holding only markup', '<p> </p><br>'],
+    ])('sends empty content for %s', (_label, description) => {
+      expect(feedFor(description)).toContain('<content type="text"/>');
+    });
+  });
 });

server/src/modules/opds/opds.service.ts

@@ -1,6 +1,7 @@
 import { Injectable, Optional } from '@nestjs/common';
 
 import { esc, fileMimeType, OPDS_MIME_ACQ, OPDS_MIME_ATOM, OPDS_MIME_NAV, OPDS_MIME_SEARCH, xmlEl, xmlLink } from './opds-xml.helpers';
+import { htmlToPlainText } from '../../common/utils/html-to-text.utils';
 import { DeliverySelectionService } from '../book/delivery-selection.service';
 import type { OpdsRequestUser } from './opds-auth.guard';
 import type { OpdsBookEntry } from './opds-book.service';
@@ -172,8 +173,12 @@ export class OpdsService {
       lines.push(`  <author>${xmlEl('name', author)}</author>`);
     }
 
-    if (book.description) {
-      lines.push(`  <content type="text">${esc(book.description)}</content>`);
+    // Descriptions are stored as HTML. Clients that honour type="text" show markup literally,
+    // and type="html" would hand unsanitized markup from book files and providers to clients
+    // that render it, so the feed carries the text alone.
+    const description = book.description ? htmlToPlainText(book.description, { preserveLineBreaks: true }) : '';
+    if (description) {
+      lines.push(`  <content type="text">${esc(description)}</content>`);
     } else {
       lines.push('  <content type="text"/>');
     }