feat: route reader by chapter id, allow duplicate-numbered chapters (0.24.0)

Real-world sources publish multiple chapters at the same number:
different scanlators ("Ch.52 from bloomingdale" + "Ch.52 from mina"),
translator notices and farewells, alt-translations. The (manga_id,
number) UNIQUE constraint from 0001 silently collapsed all of those
into a single row via the upsert path in repo::crawler. Migration 0013
drops the constraint; sync_manga_chapters now plain-INSERTs each
SourceChapterRef so every parsed chapter survives as its own row.

Identity moves from the (manga_id, number) tuple to the chapter UUID:

- `GET /api/v1/mangas/:manga_id/chapters/:chapter_id` (replaces :number)
- `GET /api/v1/mangas/:manga_id/chapters/:chapter_id/pages`
- `repo::chapter::find_by_id_in_manga` (replaces find_by_manga_and_number)
- Frontend reader route renamed to `/manga/[id]/chapter/[chapter_id]`
- Chapter links throughout (manga page list, continue-reading CTA,
  reader prev/next, history rows, bookmark cards) use chapter.id
- API clients getChapter/getChapterPages take a chapter id string

read_progress + bookmarks already FK chapter_id; they only enrich with
chapter_number for display, which is preserved.

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

backend/tests/api_uploads.rs

@@ -139,13 +139,17 @@ async fn files_endpoint_streams_in_multiple_frames(pool: PgPool) {
         .await
         .unwrap();
     assert_eq!(resp.status(), StatusCode::CREATED);
+    let chapter_id = common::body_json(resp).await["id"]
+        .as_str()
+        .unwrap()
+        .to_string();
 
     // Fetch the page back via the streaming files endpoint.
     let pages = h
         .app
         .clone()
         .oneshot(common::get(&format!(
-            "/api/v1/mangas/{manga_id}/chapters/1/pages"
+            "/api/v1/mangas/{manga_id}/chapters/{chapter_id}/pages"
         )))
         .await
         .unwrap();
@@ -317,8 +321,12 @@ async fn create_chapter_rejects_renamed_non_image_page(pool: PgPool) {
     assert_eq!(body["error"]["code"], "unsupported_media_type");
 }
 
+/// Multiple chapters can share the same number — different
+/// scanlations, re-uploads, translator notes. As of migration 0013,
+/// (manga_id, number) is not unique and each upload gets its own
+/// chapter id.
 #[sqlx::test(migrations = "./migrations")]
-async fn create_chapter_returns_409_on_duplicate_number(pool: PgPool) {
+async fn create_chapter_allows_duplicate_numbers_as_separate_chapters(pool: PgPool) {
     let h = common::harness(pool);
     let (_, cookie) = common::register_user(&h.app).await;
     let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
@@ -334,10 +342,27 @@ async fn create_chapter_returns_409_on_duplicate_number(pool: PgPool) {
     };
     let first = h.app.clone().oneshot(make()).await.unwrap();
     assert_eq!(first.status(), StatusCode::CREATED);
-    let second = h.app.oneshot(make()).await.unwrap();
-    assert_eq!(second.status(), StatusCode::CONFLICT);
-    let body = common::body_json(second).await;
-    assert_eq!(body["error"]["code"], "conflict");
+    let first_id = common::body_json(first).await["id"].as_str().unwrap().to_string();
+
+    let second = h.app.clone().oneshot(make()).await.unwrap();
+    assert_eq!(second.status(), StatusCode::CREATED);
+    let second_id = common::body_json(second).await["id"].as_str().unwrap().to_string();
+
+    assert_ne!(first_id, second_id, "each upload gets a distinct chapter id");
+
+    // List endpoint surfaces both rows.
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}/chapters")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    let items = body["items"].as_array().unwrap();
+    assert_eq!(items.len(), 2, "both Ch.1 uploads listed separately");
+    for item in items {
+        assert_eq!(item["number"], 1);
+    }
 }
 
 #[sqlx::test(migrations = "./migrations")]