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/books/page.tsx

@@ -0,0 +1,108 @@
+import { fetchBooks, searchBooks, fetchLibraries, BookDto, LibraryDto, getBookCoverUrl } from "../../lib/api";
+import { BooksGrid, EmptyState } from "../components/BookCard";
+import Link from "next/link";
+
+export const dynamic = "force-dynamic";
+
+export default async function BooksPage({
+  searchParams
+}: {
+  searchParams: Promise<{ [key: string]: string | string[] | undefined }>;
+}) {
+  const searchParamsAwaited = await searchParams;
+  const libraryId = typeof searchParamsAwaited.library === "string" ? searchParamsAwaited.library : undefined;
+  const searchQuery = typeof searchParamsAwaited.q === "string" ? searchParamsAwaited.q : "";
+  
+  const [libraries] = await Promise.all([
+    fetchLibraries().catch(() => [] as LibraryDto[])
+  ]);
+
+  let books: BookDto[] = [];
+  let nextCursor: string | null = null;
+  let searchResults: BookDto[] | null = null;
+  let totalHits: number | null = null;
+
+  if (searchQuery) {
+    // Mode recherche
+    const searchResponse = await searchBooks(searchQuery, libraryId).catch(() => null);
+    if (searchResponse) {
+      searchResults = searchResponse.hits.map(hit => ({
+        id: hit.id,
+        library_id: hit.library_id,
+        kind: hit.kind,
+        title: hit.title,
+        author: hit.author,
+        series: hit.series,
+        volume: hit.volume,
+        language: hit.language,
+        page_count: null,
+        updated_at: ""
+      }));
+      totalHits = searchResponse.estimated_total_hits;
+    }
+  } else {
+    // Mode liste
+    const booksPage = await fetchBooks(libraryId).catch(() => ({ items: [] as BookDto[], next_cursor: null }));
+    books = booksPage.items;
+    nextCursor = booksPage.next_cursor;
+  }
+
+  const displayBooks = searchResults || books;
+
+  return (
+    <>
+      <h1>Books</h1>
+      
+      {/* Filtres et recherche */}
+      <div className="card">
+        <form className="search-form">
+          <input 
+            name="q" 
+            placeholder="Search books..." 
+            defaultValue={searchQuery}
+            className="search-input"
+          />
+          <select name="library" defaultValue={libraryId || ""}>
+            <option value="">All libraries</option>
+            {libraries.map((lib) => (
+              <option key={lib.id} value={lib.id}>
+                {lib.name}
+              </option>
+            ))}
+          </select>
+          <button type="submit">Search</button>
+          {searchQuery && (
+            <Link href="/books" className="button secondary">Clear</Link>
+          )}
+        </form>
+      </div>
+
+      {/* Résultats de recherche */}
+      {searchQuery && totalHits !== null && (
+        <p className="results-info">
+          Found {totalHits} result{totalHits !== 1 ? 's' : ''} for &quot;{searchQuery}&quot;
+        </p>
+      )}
+
+      {/* Grille de livres */}
+      {displayBooks.length > 0 ? (
+        <>
+          <BooksGrid books={displayBooks} getBookCoverUrl={getBookCoverUrl} />
+          
+          {/* Pagination */}
+          {!searchQuery && nextCursor && (
+            <div className="pagination">
+              <form>
+                <input type="hidden" name="library" value={libraryId || ""} />
+                <input type="hidden" name="cursor" value={nextCursor} />
+                <button type="submit">Load more</button>
+              </form>
+            </div>
+          )}
+        </>
+      ) : (
+        <EmptyState message={searchQuery ? `No books found for "${searchQuery}"` : "No books available"} />
+      )}
+    </>
+  );
+}