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/api/src/pages.rs

@@ -351,6 +351,7 @@ async fn prefetch_page(state: AppState, params: &PrefetchParams<'_>) {
         Some(ref e) if e == "cbz" => "cbz",
         Some(ref e) if e == "cbr" => "cbr",
         Some(ref e) if e == "pdf" => "pdf",
+        Some(ref e) if e == "epub" => "epub",
         _ => return,
     }
     .to_string();
@@ -479,6 +480,7 @@ fn render_page(
         "cbz" => parsers::BookFormat::Cbz,
         "cbr" => parsers::BookFormat::Cbr,
         "pdf" => parsers::BookFormat::Pdf,
+        "epub" => parsers::BookFormat::Epub,
         _ => return Err(ApiError::bad_request("unsupported source format")),
     };