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/domain/read_progress.rs

@@ -26,6 +26,12 @@ pub struct ReadProgressSummary {
     pub chapter_number: Option<i32>,
     pub page: i32,
     pub updated_at: DateTime<Utc>,
+    /// How many chapters sit past the reader's last-read chapter (by
+    /// chapter number) — a personal "new since you last read" count.
+    /// `0` when the reader is caught up or when `chapter_number` is
+    /// unknown (manga-level progress or a deleted chapter), so we never
+    /// claim chapters are new when we can't place the reader.
+    pub new_chapters_count: i64,
 }
 
 /// Returned by `GET /me/read-progress/:manga_id`. Same shape as