docs: add comprehensive Calibre integration documentation

This commit adds complete documentation for the planned Calibre
metadata.opf sidecar file support feature.

## New Documentation

### Implementation Planning
- CALIBRE_OPF_IMPLEMENTATION.md: Detailed implementation plan with
  requirements, architecture, database mapping, and step-by-step
  implementation guide for adding Calibre metadata.opf support

### Technical Documentation
- docs/development/calibre-opf-implementation.md: Technical implementation
  details including:
  - Scanner pipeline architecture with sidecar-first approach
  - Data structures (CalibreOPFMetadata, MediaMetadata)
  - Function signatures and logic for parseCalibreMetadataOPF()
  - Database schema mapping (no changes required)
  - Testing strategy (unit and integration tests)
  - Error handling and performance considerations
  - Code examples and benchmarking approach

### User Documentation
- docs/user/calibre-integration.md: Comprehensive user guide covering:
  - What is Calibre and how Bookhoard integrates with it
  - Automatic metadata import from metadata.opf sidecar files
  - Supported metadata fields (Dublin Core + Calibre-specific)
  - Setup instructions for Calibre libraries
  - Workflow examples (fresh library, mixed library, updating metadata)
  - Troubleshooting common issues
  - Best practices for Calibre + Bookhoard workflow
  - FAQ and resources

## Updated Documentation

- README.md: Added Calibre integration feature to media management section
- docs/user/user-guide.md: Added link to Calibre integration guide
- docs/developer/development.md: Added link to Calibre implementation guide

## Feature Summary

The Calibre integration feature will allow Bookhoard to automatically
import curated metadata from Calibre's metadata.opf sidecar files,
including titles, authors, series, tags, descriptions, publishers,
identifiers (ISBN/ASIN), and contributors. Uses sidecar-first approach:
metadata.opf → embedded metadata → folder structure → filename.

All database fields already exist; no schema changes required.

docs/developer/development.md

@@ -71,6 +71,18 @@ Browse individual endpoint documentation with interactive API Explorer:
   - Event types
   - Authentication
 
+## 🛠 Implementation Guides
+
+### Calibre Integration
+
+- **[Calibre OPF Implementation](../development/calibre-opf-implementation.md)** - Technical implementation details
+  - Architecture and design
+  - Data structures and functions
+  - Database schema mapping
+  - Testing strategy
+  - Error handling
+  - Performance considerations
+
 ---
 
 **Looking for user documentation?** See the [User Portal](../user/user-guide.md)