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/indexer/src/analyzer.rs

@@ -290,6 +290,7 @@ fn book_format_from_str(s: &str) -> Option<BookFormat> {
         "cbz" => Some(BookFormat::Cbz),
         "cbr" => Some(BookFormat::Cbr),
         "pdf" => Some(BookFormat::Pdf),
+        "epub" => Some(BookFormat::Epub),
         _ => None,
     }
 }