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>

docs/peering.md

@@ -110,8 +110,15 @@ an admin can distinguish pairing from permission.
 | `GET /peer/v1/access` | Pairing state, certificate expiry and current grant status/scopes; no catalog |
 | `GET /peer/v1/catalog?cursor=N` | Up to 50 approved opaque title IDs, safe metadata, a stable work identity and cover availability |
 | `GET /peer/v1/covers/{mangaId}` | Approved cover image only; bounded to 5 MiB and charged to peer byte quota |
-| `GET /peer/v1/manga/{id}?cursor=N` | Up to 100 complete chapter IDs, public metadata and the same stable work identity |
-| `GET /peer/v1/manga/{id}/chapters/{id}` | Ordered page IDs, MIME, byte size, SHA-256 and ETag |
+| `GET /peer/v1/manga/{id}?cursor=N` | Up to 100 complete chapter IDs, public metadata, chapter `dates` and the same stable work identity |
+| `GET /peer/v1/manga/{id}/chapters/{id}` | Ordered page IDs, MIME, byte size, SHA-256 and ETag, plus chapter `dates` |
+
+Chapter `dates` are `{ ecosystem, source?, release? }`. `ecosystem` is the earliest
+date at which any version of that chapter number entered a Manga Circus library known
+to the sender. Older chapters fall back to their `added` date. The receiver stores the
+date as `x-dates.ecosystem` (`via: "peer <alias>"`) and only lets it lower an existing
+date, never raise it. See `docs/library-format.md`. Older peers omit `dates`, and the
+receiver then uses its own arrival time.
 | `GET /peer/v1/blobs/{id}` | One image, optional `Range: bytes=N-` and `If-Match`; 206 on resume |
 
 Unknown/unshared IDs return 404. Revoked serving returns 403; bad/missing grants