bugfix: chapter source key uses chapter id, not /pg-1/ (0.23.1)

Listing links point at the reader's page 1
(`.../uu/br_chapter-N/pg-1/`). The generic `derive_key_from_url` took
the last URL segment and returned `"pg-1"` for every chapter, so all
parsed chapters collapsed onto a single `chapter_sources` row downstream
and the first-manga chapter was the only row that survived. New
`derive_chapter_key_from_url` strips a trailing `/pg-\d+/` before
picking the chapter-identifying segment (`br_chapter-N` / `to_chapter-N`).

Notices, hiatus rows, and duplicate-numbered chapters are preserved as
distinct parser entries. The (manga_id, number) UNIQUE collapse in the
chapters table is a separate, follow-up concern handled in
feat/chapter-id-routing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.23.0",
+  "version": "0.23.1",
   "private": true,
   "type": "module",
   "scripts": {