Implements GetArtistSettings, SaveArtistSettings, GetAllArtistSettings,
and UpdateArtistSettings with full test coverage (10 new tests).
All 17 database tests pass.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add internal/database package with SQLite3 connection management,
versioned migration system, and three schema tables (artist_settings,
external_releases, notifications_sent). Includes 7 tests covering
initialization, migration idempotency, Close() behavior, and schema
validation using in-memory SQLite.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
