feat: add series support for book organization

API:
- Add /libraries/{id}/series endpoint to list series with book counts
- Add series filter to /books endpoint
- Fix SeriesItem to return first_book_id properly (using CTE with ROW_NUMBER)

Indexer:
- Parse series from parent folder name relative to library root
- Store series in database when indexing books

Backoffice:
- Add Books page with grid view, search, and pagination
- Add Series page showing series with cover images
- Add Library books page filtered by series
- Add book detail page
- Add Series column in libraries list with clickable link
- Create BookCard component for reusable book display
- Add CSS styles for books grid, series grid, and book details
- Add proxy API route for book cover images (fixing CORS issues)

Parser:
- Add series field to ParsedMetadata
- Extract series from file path relative to library root

Books without a parent folder are categorized as 'unclassified' series.

apps/backoffice/app/components/BookCard.tsx

@@ -0,0 +1,70 @@
+import Image from "next/image";
+import Link from "next/link";
+import { BookDto } from "../../lib/api";
+
+interface BookCardProps {
+  book: BookDto;
+  getBookCoverUrl: (bookId: string) => string;
+}
+
+export function BookCard({ book, getBookCoverUrl }: BookCardProps) {
+  return (
+    <Link href={`/books/${book.id}`} className="book-card">
+      <div className="book-cover">
+        <Image
+          src={getBookCoverUrl(book.id)}
+          alt={`Cover of ${book.title}`}
+          width={150}
+          height={220}
+          className="cover-image"
+          unoptimized
+        />
+      </div>
+      <div className="book-info">
+        <h3 className="book-title" title={book.title}>
+          {book.title}
+        </h3>
+        {book.author && (
+          <p className="book-author">{book.author}</p>
+        )}
+        {book.series && (
+          <p className="book-series">
+            {book.series}
+            {book.volume && ` #${book.volume}`}
+          </p>
+        )}
+        <div className="book-meta">
+          <span className={`book-kind ${book.kind}`}>{book.kind.toUpperCase()}</span>
+          {book.language && <span className="book-lang">{book.language.toUpperCase()}</span>}
+        </div>
+      </div>
+    </Link>
+  );
+}
+
+interface BooksGridProps {
+  books: BookDto[];
+  getBookCoverUrl: (bookId: string) => string;
+}
+
+export function BooksGrid({ books, getBookCoverUrl }: BooksGridProps) {
+  return (
+    <div className="books-grid">
+      {books.map((book) => (
+        <BookCard key={book.id} book={book} getBookCoverUrl={getBookCoverUrl} />
+      ))}
+    </div>
+  );
+}
+
+interface EmptyStateProps {
+  message: string;
+}
+
+export function EmptyState({ message }: EmptyStateProps) {
+  return (
+    <div className="empty-state">
+      <p>{message}</p>
+    </div>
+  );
+}