fix(history): count distinct new chapter numbers, authoritative on detail

new_chapters_count over-counted when scanlations share a chapter number
((manga_id, number) is non-unique, migration 0013): COUNT(*) tallied rows,
so duplicate-numbered chapters inflated the badge. Switch both the list and
per-manga read-progress queries to COUNT(DISTINCT number), and drop the dead
COALESCE (an empty set counts as 0, and the NULL-number case is handled by
the predicate). Fix the misleading comment.

Expose new_chapters_count on GET /me/read-progress/:manga_id and drive the
detail page's "N new" badge from it, instead of recomputing over the
paginated chapter list — which under-counted series past 50 chapters and
disagreed with the homepage shelf. Read markers stay client-side over the
loaded chapters and are documented as by-number (all we persist).

Adds duplicate-scanlation tests at both endpoints.

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

frontend/src/lib/api/read_progress.ts

@@ -79,6 +79,10 @@ export type ReadProgressForManga = {
     chapter_number: number | null;
     page: number;
     updated_at: string;
+    /** Distinct chapter numbers past the last-read chapter — the
+     * authoritative "new since last read" count (over all chapters, not the
+     * detail page's paginated list). `0` when caught up / position unknown. */
+    new_chapters_count: number;
 };
 
 /**

frontend/src/lib/chapterProgress.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { isChapterRead, countNewChapters } from './chapterProgress';
+import { isChapterRead } from './chapterProgress';
 
 describe('isChapterRead', () => {
     it('marks chapters at or before the last-read number as read', () => {
@@ -15,19 +15,3 @@ describe('isChapterRead', () => {
         expect(isChapterRead(1, null)).toBe(false);
     });
 });
-
-describe('countNewChapters', () => {
-    const chapters = [{ number: 1 }, { number: 2 }, { number: 3 }];
-
-    it('counts chapters past the last-read number', () => {
-        expect(countNewChapters(chapters, 1)).toBe(2);
-    });
-
-    it('is zero when caught up to the latest', () => {
-        expect(countNewChapters(chapters, 3)).toBe(0);
-    });
-
-    it('is zero when the last-read position is unknown', () => {
-        expect(countNewChapters(chapters, null)).toBe(0);
-    });
-});

frontend/src/lib/chapterProgress.ts

@@ -1,8 +1,6 @@
-// Pure helpers for reflecting a reader's personal progress on a manga's
-// chapter list. Kept UI-free so the read/unread and "new since last read"
-// logic is unit-testable without rendering the detail page.
-
-export type NumberedChapter = { number: number };
+// Pure helper for reflecting a reader's personal progress on a manga's
+// chapter list. Kept UI-free so the read/unread logic is unit-testable
+// without rendering the detail page.
 
 /**
  * A chapter counts as read once its number is at or before the reader's
@@ -13,16 +11,7 @@ export function isChapterRead(chapterNumber: number, lastReadNumber: number | nu
     return lastReadNumber != null && chapterNumber <= lastReadNumber;
 }
 
-/**
- * How many of the given chapters sit past the reader's last-read chapter
- * (by number) — the personal "new since you last read" count. `0` when the
- * last-read position is unknown. Counts over the provided (loaded) list, so
- * for a paginated chapter list it reflects what's currently shown.
- */
-export function countNewChapters(
-    chapters: NumberedChapter[],
-    lastReadNumber: number | null
-): number {
-    if (lastReadNumber == null) return 0;
-    return chapters.reduce((n, c) => (c.number > lastReadNumber ? n + 1 : n), 0);
-}
+// Note: the "new since last read" *count* is computed server-side
+// (distinct chapter numbers over ALL chapters — see
+// repo::read_progress) and delivered on the read-progress payloads, so it
+// isn't recomputed here over the detail page's paginated chapter list.