fix: upload retry reuses the created manga instead of duplicating it

After a partial upload failure (manga created, a chapter failed), re-submitting
re-ran createManga and spawned a duplicate manga. Track the created manga id:
a retry skips creation, re-sends only the not-yet-done chapters, disables the
now-saved title, relabels the button "Retry failed chapters", and shows a note.

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

frontend/e2e/upload-retry.spec.ts

@@ -0,0 +1,104 @@
+import { test, expect, type Page } from './fixtures';
+
+// A partial upload failure (manga created, a chapter fails) must NOT create a
+// duplicate manga when the user retries — the second submit reuses the created
+// manga and only re-sends the failed chapter.
+
+const userFixture = {
+    id: 'u1',
+    username: 'uploader',
+    created_at: '2026-01-01T00:00:00Z',
+    is_admin: false
+};
+
+const manga = {
+    id: 'm9',
+    title: 'Retry Saga',
+    status: 'ongoing',
+    description: null,
+    authors: [],
+    alt_titles: [],
+    genres: [],
+    cover_image_path: null,
+    created_at: '2026-01-01T00:00:00Z',
+    updated_at: '2026-01-01T00:00:00Z'
+};
+
+async function stub(page: Page, counters: { mangaPosts: number; chapterPosts: number }) {
+    await page.route('**/api/v1/auth/config', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: JSON.stringify({ self_register_enabled: true, private_mode: false }) })
+    );
+    await page.route('**/api/v1/auth/me', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: JSON.stringify({ user: userFixture }) })
+    );
+    await page.route('**/api/v1/auth/me/preferences', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: '{}' })
+    );
+    await page.route('**/api/v1/genres', (r) => r.fulfill({ status: 200, contentType: 'application/json', body: '[]' }));
+
+    await page.route('**/api/v1/mangas', (r) => {
+        if (r.request().method() === 'POST') {
+            counters.mangaPosts += 1;
+            return r.fulfill({ status: 201, contentType: 'application/json', body: JSON.stringify(manga) });
+        }
+        return r.fallback();
+    });
+    // Chapter POST: fail the first attempt, succeed on the retry.
+    await page.route('**/api/v1/mangas/m9/chapters', (r) => {
+        if (r.request().method() === 'POST') {
+            counters.chapterPosts += 1;
+            if (counters.chapterPosts === 1) {
+                return r.fulfill({
+                    status: 500,
+                    contentType: 'application/json',
+                    body: JSON.stringify({ error: { code: 'internal_error', message: 'boom' } })
+                });
+            }
+            return r.fulfill({
+                status: 201,
+                contentType: 'application/json',
+                body: JSON.stringify({ id: 'c1', manga_id: 'm9', number: 1, title: null, page_count: 1, created_at: '2026-01-01T00:00:00Z' })
+            });
+        }
+        return r.fallback();
+    });
+    // Post-success destination + its data.
+    await page.route('**/api/v1/mangas/m9', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: JSON.stringify(manga) })
+    );
+    await page.route('**/api/v1/mangas/m9/chapters?*', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: JSON.stringify({ items: [], page: { limit: 200, offset: 0, total: 0 } }) })
+    );
+    await page.route('**/api/v1/me/bookmarks*', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: JSON.stringify({ items: [], page: { limit: 50, offset: 0, total: 0 } }) })
+    );
+    await page.route('**/api/v1/me/read-progress/m9', (r) =>
+        r.fulfill({ status: 404, contentType: 'application/json', body: JSON.stringify({ error: { code: 'not_found', message: 'x' } }) })
+    );
+}
+
+test('retrying a partially-failed upload does not create a duplicate manga', async ({ page }) => {
+    const counters = { mangaPosts: 0, chapterPosts: 0 };
+    await stub(page, counters);
+
+    await page.goto('/upload');
+    await page.getByTestId('manga-title').fill('Retry Saga');
+    await page.getByTestId('add-chapter').click();
+    const png = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);
+    await page
+        .getByTestId('staged-chapter-pages-input')
+        .setInputFiles([{ name: 'p.png', mimeType: 'image/png', buffer: png }]);
+
+    // First submit: manga created, chapter fails → the retry note + relabeled button.
+    await page.getByTestId('manga-submit').click();
+    await expect(page.getByTestId('manga-success')).toBeVisible();
+    await expect(page.getByTestId('manga-created-note')).toBeVisible();
+    await expect(page.getByTestId('manga-submit')).toHaveText('Retry failed chapters');
+    expect(counters.mangaPosts).toBe(1);
+
+    // Retry: no second manga POST, chapter re-sent, navigate to the manga.
+    await page.getByTestId('manga-submit').click();
+    await expect(page).toHaveURL(/\/manga\/m9$/);
+    expect(counters.mangaPosts).toBe(1);
+    expect(counters.chapterPosts).toBe(2);
+});