fix(stats): preserve historical KOReader completion dates

Use the source finish day when importing completed KOReader books so
completion timelines reflect reading history instead of the sync date.
Preserve newer manual edits and retain fallback behavior for unusable dates.

Closes #1783

server/test/reading-attempts.e2e-spec.ts

@@ -117,6 +117,79 @@ describe('Reading attempts main-flow simulation (docker e2e)', { timeout: TIMEOU
     expect(history.items[0]?.outcome).toBe('completed');
   });
 
+  it('uses a historical KOReader finish day for the book and completion statistics without overwriting later manual edits', async () => {
+    const book = await createBook();
+    const fileHash = randomUUID().replaceAll('-', '');
+    await ctx.db.update(schema.bookFiles).set({ fileHash }).where(eq(schema.bookFiles.id, book.fileId));
+    const username = `attempt-date-${randomUUID()}`;
+    const password = 'HistoricalFinish123';
+    const credentials = await ctx.app.inject({
+      method: 'POST',
+      url: '/api/v1/koreader/credentials',
+      headers: auth(ctx.adminToken),
+      payload: { username, password },
+    });
+    expect(credentials.statusCode).toBe(201);
+
+    const year = new Date().getUTCFullYear() - 1;
+    const finishedOn = `${year}-08-22`;
+    const startTime = Date.parse(`${finishedOn}T18:00:00.000Z`) / 1000;
+    const deviceFields = { deviceId: 'historical-finish-e2e', deviceModel: 'E2E', pluginVersion: '1.7.0' };
+    const deviceHeaders = { 'x-auth-user': username, 'x-auth-key': password };
+    const stats = await ctx.app.inject({
+      method: 'POST',
+      url: '/api/v1/koreader/plugin/page-stats',
+      headers: deviceHeaders,
+      payload: {
+        ...deviceFields,
+        books: [{ hash: fileHash, events: [{ page: 100, totalPages: 100, startTime, durationSeconds: 60 }] }],
+      },
+    });
+    expect(stats.statusCode).toBe(201);
+    expect(stats.json().results[0].accepted).toBe(1);
+
+    const timeline = async () => {
+      const response = await ctx.app.inject({
+        method: 'GET',
+        url: '/api/v1/user-statistics/completion-timeline?days=3650',
+        headers: auth(ctx.adminToken),
+      });
+      expect(response.statusCode).toBe(200);
+      return response.json() as Array<{ year: number; month: number; count: number }>;
+    };
+    const before = await timeline();
+    const uploadState = () =>
+      ctx.app.inject({
+        method: 'POST',
+        url: '/api/v1/koreader/plugin/book-states',
+        headers: deviceHeaders,
+        payload: { ...deviceFields, books: [{ hash: fileHash, status: 'complete', statusModified: finishedOn }] },
+      });
+    expect((await uploadState()).statusCode).toBe(201);
+    expect((await uploadState()).statusCode).toBe(201);
+
+    const history = await listAttempts(book.bookId);
+    expect(history.total).toBe(1);
+    expect(history.items[0]).toMatchObject({ outcome: 'completed', endedOn: finishedOn });
+    const detail = await ctx.app.inject({ method: 'GET', url: `/api/v1/books/${book.bookId}`, headers: auth(ctx.adminToken) });
+    expect(detail.statusCode).toBe(200);
+    expect(detail.json().readStatus.finishedAt).toBe(`${finishedOn}T00:00:00.000Z`);
+    const after = await timeline();
+    expect(after.find((point) => point.year === year && point.month === 8)?.count).toBe(
+      (before.find((point) => point.year === year && point.month === 8)?.count ?? 0) + 1,
+    );
+    const now = new Date();
+    expect(after.find((point) => point.year === now.getUTCFullYear() && point.month === now.getUTCMonth() + 1)?.count).toBe(
+      before.find((point) => point.year === now.getUTCFullYear() && point.month === now.getUTCMonth() + 1)?.count,
+    );
+
+    expect((await patchStatus(book.bookId, { finishedAt: `${year}-08-23` })).statusCode).toBe(200);
+    expect((await uploadState()).statusCode).toBe(201);
+    const editedHistory = await listAttempts(book.bookId);
+    expect(editedHistory.total).toBe(1);
+    expect(editedHistory.items[0]?.endedOn).toBe(`${year}-08-23`);
+  });
+
   it('3. starts a manual reread and resets progress without deleting history', async () => {
     const book = await createBook();
     await patchStatus(book.bookId, { status: 'read', startedAt: '2024-01-01', finishedAt: '2024-01-10' });