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/layout.tsx

@@ -25,6 +25,7 @@ export default function RootLayout({ children }: { children: ReactNode }) {
             <div className="links-wrap">
               <div className="links">
                 <Link href="/">Dashboard</Link>
+                <Link href="/books">Books</Link>
                 <Link href="/libraries">Libraries</Link>
                 <Link href="/jobs">Jobs</Link>
                 <Link href="/tokens">Tokens</Link>