feat(history): report personal new-chapter count on read-progress list

Add `new_chapters_count` to each `GET /me/read-progress` item: how many
chapters sit past the reader's last-read chapter, computed by a correlated
subquery on chapter number. It is 0 when the reader is caught up, and 0
when the last-read chapter is unknown (manga-level progress or a deleted
chapter) so we never over-claim. (manga_id, number) is non-unique, so this
counts positions past the reader rather than distinct files.

Backs the personal "new since last read" indicator on the upcoming
Continue-reading shelf. Tests cover the mid-series, caught-up, and
no-read-chapter cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/repo/read_progress.rs

@@ -132,7 +132,21 @@ pub async fn list_for_user(
                rp.chapter_id,
                c.number            AS chapter_number,
                rp.page,
-               rp.updated_at
+               rp.updated_at,
+               -- Personal "new since last read": chapters numbered past the
+               -- reader's last-read chapter. `c.number` is NULL for
+               -- manga-level progress or a deleted chapter, in which case
+               -- the guard yields 0 rather than counting every chapter.
+               -- (manga_id, number) is non-unique — translations share a
+               -- number — so this counts positions past the reader, not
+               -- distinct files.
+               COALESCE((
+                   SELECT count(*)
+                     FROM chapters c2
+                    WHERE c2.manga_id = rp.manga_id
+                      AND c.number IS NOT NULL
+                      AND c2.number > c.number
+               ), 0)               AS new_chapters_count
           FROM read_progress rp
           JOIN mangas m ON m.id = rp.manga_id
           LEFT JOIN chapters c ON c.id = rp.chapter_id