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>

public/settings.js

@@ -38,6 +38,11 @@
     h('option', { value: 'dim' }, 'Dim (70% brightness)'),
     h('option', { value: 'extra-dim' }, 'Extra dim (45% brightness)'));
   tone.value = S.pageTone;
+  const readDate = h('select', { onchange: e => save({ readDate: e.target.value }) },
+    h('option', { value: 'hour' }, 'Date and hour (e.g. 23 Oct, 21h)'),
+    h('option', { value: 'date' }, 'Date only'),
+    h('option', { value: 'off' }, 'Do not show'));
+  readDate.value = S.readDate;
   const LABEL = { flow: 'Flow', page: 'Page', book: 'Book', strip: 'Webtoon flow' };
   const zooms = () => Object.keys(LABEL).map(k => `${LABEL[k]} ${Math.round((S.zoom[k] || 1) * 100)}%`).join(' · ');
   const zoomText = h('span', { class: 'muted' }, zooms());
@@ -85,6 +90,13 @@
       h('button', { class: 'btn small', onclick: async () => { await save({ zoom: {} }); zoomText.textContent = zooms(); } }, 'Reset to fit'),
       h('div', { class: 'muted small' }, '100% = the page fits the screen. Change it in the reader with Ctrl + mouse wheel, a pinch, or the + / - keys.'))));
 
+  section('user/reading').append(h('section', { class: 'card' },
+    h('h2', {}, 'Chapter list dates'),
+    h('label', { class: 'field' }, h('span', {}, 'When I read a chapter'), readDate,
+      h('div', { class: 'muted small' }, 'Shown next to ✓ in the chapter list: when you first finished the chapter, in your local time. Marking it unread clears it.')),
+    check('releaseDate', 'Show when each chapter was released',
+      'Best known date first: the official release, else the upload at the source (≈), else when it first reached Manga Circus (◎), else when it was added here (↓). Hover a date for all known dates.')));
+
   const readingHoursBody = h('div', {}, h('p', { class: 'muted' }, 'Loading server reading hours…'));
   section('user/reading-hours').append(h('section', { class: 'card' },
     h('h2', {}, 'Server reading hours'), readingHoursBody,