test: add integration tests and performance benchmarks

Added comprehensive test suite for Sprint 2 validation:

Integration Tests (library_integration_test.go - 470 lines):
- TestIntegration_ScanToLibraryFlow: Complete scan → database → library flow
- TestIntegration_MetadataFetchToLibrary: Metadata fetch → storage → display
- TestIntegration_SearchFiltering: Search and filter functionality
- TestIntegration_DuplicateDetection: Indexed duplicate detection
- TestIntegration_FavoriteManagement: Favorite marking and filtering
- TestIntegration_MetadataRegistry: Fetcher integration

Performance Benchmarks (scanner_bench_test.go - 410 lines):
- BenchmarkScanDirectories: 100, 1K, 10K file sets
- BenchmarkDuplicateDetection: 100, 1K, 10K model databases
- BenchmarkFileTypeDetection: Extension matching speed
- BenchmarkMetadataExtraction: Filename parsing speed
- BenchmarkDatabaseUpsert: Insert performance
- BenchmarkDatabaseQuery: Indexed query performance with 10K models
- BenchmarkCompleteWorkflow: End-to-end scan performance

These tests validate:
- O(log n) duplicate detection performance
- Complete user workflows
- Database integration
- Search and filter accuracy
- Metadata preservation

Sprint 2 Days 12-13 complete.

Author: claude