feat: Hardcover→Directus book sync + fix empty states for projects/books

- Add POST /api/n8n/hardcover/sync-books — n8n calls this after detecting
  finished books in Hardcover. Authenticates via N8N_SECRET_TOKEN/N8N_API_KEY,
  deduplicates by hardcover_id, creates new book_reviews entries in Directus.

- Add getBookReviewByHardcoverId() + createBookReview() to lib/directus.ts.
  Check uses GraphQL filter; create uses Directus REST POST /items/book_reviews.

- ReadBooks: replace silent return null with a visible empty state so the
  section stays visible with a hint until the n8n sync populates it.

- Projects: add "No projects yet." placeholder instead of blank grid when
  both Directus and PostgreSQL return no data.

- Add home.about.readBooks.empty i18n key (EN + DE).

n8n workflow setup:
  Schedule → HTTP Hardcover GraphQL (books_read) → Code (transform) →
  POST /api/n8n/hardcover/sync-books with array of { hardcover_id, title,
  author, image, rating, finished_at }

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/n8n/hardcover/sync-books/route.ts

@@ -0,0 +1,125 @@
+/**
+ * POST /api/n8n/hardcover/sync-books
+ *
+ * Called by an n8n workflow whenever books are finished in Hardcover.
+ * Creates new entries in the Directus book_reviews collection.
+ * Deduplicates by hardcover_id — safe to call repeatedly.
+ *
+ * n8n Workflow setup:
+ *   1. Schedule Trigger (every hour)
+ *   2. HTTP Request → Hardcover GraphQL (query: me { books_read(limit: 20) { ... } })
+ *   3. Code Node → transform to array of HardcoverBook objects
+ *   4. HTTP Request → POST https://dk0.dev/api/n8n/hardcover/sync-books
+ *      Headers: Authorization: Bearer <N8N_SECRET_TOKEN>
+ *      Body: [{ hardcover_id, title, author, image, rating, finished_at }, ...]
+ *
+ * Expected body shape (array or single object):
+ *   {
+ *     hardcover_id: string | number   // Hardcover book ID, used for deduplication
+ *     title: string
+ *     author: string
+ *     image?: string                  // Cover image URL
+ *     rating?: number                 // 1–5
+ *     finished_at?: string            // ISO date string
+ *   }
+ */
+
+import { NextRequest, NextResponse } from 'next/server';
+import { getBookReviewByHardcoverId, createBookReview } from '@/lib/directus';
+import { checkRateLimit, getClientIp } from '@/lib/auth';
+
+export const runtime = 'nodejs';
+export const dynamic = 'force-dynamic';
+
+interface HardcoverBook {
+  hardcover_id: string | number;
+  title: string;
+  author: string;
+  image?: string;
+  rating?: number;
+  finished_at?: string;
+}
+
+export async function POST(request: NextRequest) {
+  // Auth: require N8N_SECRET_TOKEN or N8N_API_KEY
+  const authHeader = request.headers.get('Authorization');
+  const apiKeyHeader = request.headers.get('X-API-Key');
+  const validToken = process.env.N8N_SECRET_TOKEN;
+  const validApiKey = process.env.N8N_API_KEY;
+
+  const isAuthenticated =
+    (validToken && authHeader === `Bearer ${validToken}`) ||
+    (validApiKey && apiKeyHeader === validApiKey);
+
+  if (!isAuthenticated) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  // Rate limit: max 10 sync requests per minute
+  const ip = getClientIp(request);
+  if (!checkRateLimit(ip, 10, 60000, 'hardcover-sync')) {
+    return NextResponse.json({ error: 'Rate limit exceeded' }, { status: 429 });
+  }
+
+  let books: HardcoverBook[];
+  try {
+    const body = await request.json();
+    books = Array.isArray(body) ? body : [body];
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 });
+  }
+
+  if (books.length === 0) {
+    return NextResponse.json({ success: true, created: 0, skipped: 0, errors: 0 });
+  }
+
+  const results = {
+    created: 0,
+    skipped: 0,
+    errors: 0,
+    details: [] as string[],
+  };
+
+  for (const book of books) {
+    if (!book.title || !book.author) {
+      results.errors++;
+      results.details.push(`Skipped (missing title/author): ${JSON.stringify(book).slice(0, 80)}`);
+      continue;
+    }
+
+    const hardcoverId = String(book.hardcover_id);
+
+    // Deduplication: skip if already in Directus
+    const existing = await getBookReviewByHardcoverId(hardcoverId);
+    if (existing) {
+      results.skipped++;
+      results.details.push(`Skipped (exists): "${book.title}"`);
+      continue;
+    }
+
+    // Create new entry in Directus
+    const created = await createBookReview({
+      hardcover_id: hardcoverId,
+      book_title: book.title,
+      book_author: book.author,
+      book_image: book.image,
+      rating: book.rating,
+      finished_at: book.finished_at,
+      status: 'published',
+    });
+
+    if (created) {
+      results.created++;
+      results.details.push(`Created: "${book.title}" → id=${created.id}`);
+    } else {
+      results.errors++;
+      results.details.push(`Error creating: "${book.title}" (Directus unavailable or token missing)`);
+    }
+  }
+
+  if (process.env.NODE_ENV === 'development') {
+    console.log('[sync-books]', results);
+  }
+
+  return NextResponse.json({ success: true, source: 'directus', ...results });
+}