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>

reading-analytics.js

@@ -115,6 +115,17 @@ function chapterInsight(record) {
   };
 }
 
+// Earliest completion time the clock still remembers (only the last few visits are kept).
+// approx: older completed passes were already dropped, so the first read may be earlier.
+function earliestCompletion(record) {
+  if (!record || typeof record !== 'object') return null;
+  const passes = [record.currentPass, ...(record.recentSessions || []).map(item => item && item.pass)];
+  const times = passes.map(pass => pass && whole(pass.completedAt)).filter(time => time > 0);
+  if (!times.length) return null;
+  const counted = new Set(times).size;
+  return { at: Math.min(...times), approx: whole(record.completedPasses) > counted };
+}
+
 function median(values) {
   const sorted = values.filter(value => Number.isFinite(value) && value > 0).sort((a, b) => a - b);
   if (!sorted.length) return null;
@@ -295,6 +306,6 @@ function applyPulse(record, pulse, { version, pageCount, eligible, now = Date.no
 }
 
 module.exports = {
-  INPUT_TYPES, applyPulse, applyVisit, chapterInsight, compactRanges, eligiblePages, emptyRecord,
+  INPUT_TYPES, applyPulse, applyVisit, chapterInsight, compactRanges, earliestCompletion, eligiblePages, emptyRecord,
   libraryInsight, mangaInsight, publicSummary, rangeSet,
 };