Implement sidecar-first metadata extraction approach that prioritizes
Calibre metadata.opf files over embedded metadata when available.
Key Features:
- Sidecar-first approach: Check for metadata.opf before extracting embedded
- Full Dublin Core namespace support: Use complete namespace URLs
- Calibre-specific meta tags: Extract series, series_index from <meta> tags
- Graceful degradation: Fall back to embedded metadata on parse failure
- Identifier extraction: Support ISBN and ASIN from Dublin Core identifiers
- Date parsing: Handle ISO 8601 timestamps and simple date formats
Implementation Details:
- Added extractCalibreSidecar() to check for and parse metadata.opf
- Added parseCalibreMetadataOPF() with full Dublin Core namespace handling
- Modified extractMetadata() to try sidecar first, fallback to embedded
- Added CalibreOPFMetadata struct for intermediate parsing
- Cover image support: findSidecarCover() for sidecar metadata
Tests:
- Unit tests for parseCalibreMetadataOPF() with real Calibre file examples
- Integration tests for Calibre library scanning
This allows users with Calibre-managed libraries to import their curated
metadata (series, tags, custom covers) into Bookhoard.
Fixes: #calibre-opf-support
