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>

infra/migrations/0046_add_epub_format.sql

@@ -0,0 +1,10 @@
+-- Add EPUB to allowed format values in book_files and books tables.
+-- PostgreSQL CHECK constraints are dropped+recreated (no ALTER CONSTRAINT).
+
+-- book_files.format
+ALTER TABLE book_files DROP CONSTRAINT IF EXISTS book_files_format_check;
+ALTER TABLE book_files ADD CONSTRAINT book_files_format_check CHECK (format IN ('pdf', 'cbz', 'cbr', 'epub'));
+
+-- books.format (denormalized column added in 0020)
+ALTER TABLE books DROP CONSTRAINT IF EXISTS books_format_check;
+ALTER TABLE books ADD CONSTRAINT books_format_check CHECK (format IN ('pdf', 'cbz', 'cbr', 'epub'));