feat: add EPUB format support with spine-aware image extraction

Parse EPUB structure (container.xml → OPF → spine → XHTML) to extract
images in reading order. Zero new dependencies — reuses zip + regex
crates with pre-compiled regexes and per-file index cache for
performance. Falls back to CBZ-style image listing when spine contains
no images. Includes DB migration, API/indexer/backoffice updates.

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

apps/backoffice/app/components/BookCard.tsx

@@ -115,6 +115,7 @@ export function BookCard({ book, readingStatus }: BookCardProps) {
               ${(book.format ?? book.kind) === 'cbz' ? 'bg-success/10 text-success' : ''}
               ${(book.format ?? book.kind) === 'cbr' ? 'bg-warning/10 text-warning' : ''}
               ${(book.format ?? book.kind) === 'pdf' ? 'bg-destructive/10 text-destructive' : ''}
+              ${(book.format ?? book.kind) === 'epub' ? 'bg-info/10 text-info' : ''}
             `}>
               {book.format ?? book.kind}
             </span>