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/user/user-guide.md

@@ -27,6 +27,17 @@ Learn how to configure your e-reader devices to sync with Bookhoard:
   - Conflict resolution
   - Best practices
 
+## 📚 Calibre Integration
+
+- **[Calibre Integration Guide](calibre-integration.md)** - Integrating with Calibre libraries
+  - What is Calibre?
+  - Automatic metadata import
+  - Setting up your Calibre library
+  - Supported metadata and file types
+  - Workflow examples
+  - Troubleshooting
+  - Best practices
+
 ## 🎨 Frontend Guide
 
 **[Frontend Guide](frontend-guide.md)** - Learn how to use the Bookhoard web interface