Add .epub and .pdf to comics type, add .pdf to manga type, and ensure
avif/tiff/tif extensions are consistently included in manga across all
layers. Update test fixtures to match the canonical extension lists.
Rename test helper files from .go to _test.go suffix to comply with
Go testing conventions. This ensures proper test file recognition by
the Go toolchain and improves build organization.
- library_test_comprehensive.go → library_test_comprehensive_test.go
- test_helpers.go → test_helpers_test.go
- test_helpers_db.go → test_helpers_db_test.go
john-okeefe