Implements ScanArtist/ScanAll in internal/scanner loading local/external
releases via the database layer with ctx-cancellation checks, plus a
compute-only run() hook that logs missing-release counts. Add table-driven
tests using in-memory SQLite fixtures.
Add FindMissingReleases and MissingRelease to internal/scanner: skips
IsIgnored external releases, scopes matches per ArtistID, and reports
external releases with no local album above the fuzzy threshold.
