Move completion checks into metadata_service and make them follow the configured metadata source priority.
Drop the old test-mode path, remove the web_server wrapper indirection, and keep artist inference on explicit release metadata instead of guessing from a track search.
Add coverage for the source-priority completion behavior and the safer artist-name handling.
Move artist discography resolution into core metadata_service, introduce MetadataLookupOptions, and keep web_server focused on request handling. Add focused tests for the new service boundary and preserve current fallback behavior for now.
