Chapter dates: read date setting and tiered release dates (BL-0047)

- Chapter rows show when you first finished a chapter (setting: date + hour,
  date, off) from a new durable progress doneAt; older reads fall back to the
  reading clock's earliest remembered completion.
- chapter.json keeps added (arrival in this library, now kept on replace) plus
  x-dates.source (source upload, bulk imports flagged), x-dates.release
  (official, for a later metadata job) and x-dates.ecosystem (first entry into
  any Manga Circus library; carried over peering, earliest wins, clamped).
- Rows show one release date by tier: official > source upload > ecosystem >
  library, with all known dates in the tooltip (setting to hide).
- Fixes: WeebCentral dates came from the chapter page, which has none (every
  chapter got the download time); they now come from the full chapter list.
  MangaDex no longer writes a group's publishAt into added. Peering sends dates.
- tools/backfill-chapter-dates.js migrates an existing library (dry run default).
- Coverage dry run on the server: docs/chapter-dates-coverage-2026-10-24.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scraper/test/source-plugin.test.js

@@ -10,8 +10,8 @@ const { adapters, adapterFor } = require('../adapters');
 const seriesUrl = 'https://weebcentral.com/series/ABC/title';
 const chapterUrl = 'https://weebcentral.com/chapters/XYZ';
 const seriesHtml = '<h1>Test Manga</h1><div hx-get="/series/ABC/full-chapter-list"></div>';
-const listHtml = '<a href="/chapters/XYZ">Chapter 1</a><a href="/chapters/LMN">Log. 2</a>';
-const chapterHtml = '<time datetime="2026-01-01"></time>';
+const listHtml = '<a href="/chapters/XYZ"><span>Chapter 1</span><time datetime="2026-01-01T15:06:05.348Z">x</time></a><a href="/chapters/LMN">Log. 2</a>';
+const chapterHtml = '<div>no dates on the chapter page</div>';
 const imagesHtml = '<img src="https://img.example/1.jpg"><img data-src="https://img.example/2.jpg">';
 
 test('first-party registry rejects unknown plugins and malformed input', () => {
@@ -45,7 +45,9 @@ test('in-process parser preserves WeebCentral catalog and page discovery', async
   assert.deepEqual(catalog.chapters.map(chapter => chapter.number), ['1', '2']);
   const chapter = await discoverWeebChapter(catalog.chapters[0], http);
   assert.equal(chapter.pages.length, 2);
-  assert.equal(chapter.added, '2026-01-01');
+  assert.equal(chapter.added, undefined);
+  assert.deepEqual(chapter.dates, { source: { at: '2026-01-01T15:06:05.348Z', name: 'WeebCentral', batch: false } });
+  assert.equal(catalog.chapters[1].uploadedAt, undefined);
   assert.equal(requested.length, 4);
 });