Implement MarkNotificationSent, IsNotificationSent, and
GetUnnotifiedReleases with LEFT JOIN query. Includes 10
table-driven tests covering success, error, idempotent,
and edge cases. Also go fmt cleanup on existing files.
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>
