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/search.rs

@@ -47,7 +47,7 @@ pub struct SearchResponse {
     params(
         ("q" = String, Query, description = "Search query (books + series via PostgreSQL full-text)"),
         ("library_id" = Option<String>, Query, description = "Filter by library ID"),
-        ("type" = Option<String>, Query, description = "Filter by type (cbz, cbr, pdf)"),
+        ("type" = Option<String>, Query, description = "Filter by type (cbz, cbr, pdf, epub)"),
         ("kind" = Option<String>, Query, description = "Filter by kind (alias for type)"),
         ("limit" = Option<usize>, Query, description = "Max results per type (max 100)"),
     ),