feat: add CRUD operations for artist_settings table

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>

docs/plans/2026-05-20-foundation-layer.md

@@ -87,14 +87,14 @@ Build the foundation layer of NaviWatcher: a greenfield Go project with zero exi
 - [x] run tests — must pass before task 4
 
 ### Task 4: Database layer — CRUD operations for artist_settings
-- [ ] implement `GetArtistSettings(db *DB, id string) (*ArtistSettings, error)`
-- [ ] implement `SaveArtistSettings(db *DB, settings *ArtistSettings) error`
-- [ ] implement `GetAllArtistSettings(db *DB) ([]ArtistSettings, error)`
-- [ ] implement `UpdateArtistSettings(db *DB, id string, updates map[string]interface{}) error`
-- [ ] write tests for GetArtistSettings (found and not found cases)
-- [ ] write tests for SaveArtistSettings (insert and update)
-- [ ] write tests for GetAllArtistSettings (empty and populated)
-- [ ] run tests — must pass before task 5
+- [x] implement `GetArtistSettings(db *DB, id string) (*ArtistSettings, error)`
+- [x] implement `SaveArtistSettings(db *DB, settings *ArtistSettings) error`
+- [x] implement `GetAllArtistSettings(db *DB) ([]ArtistSettings, error)`
+- [x] implement `UpdateArtistSettings(db *DB, id string, updates map[string]interface{}) error`
+- [x] write tests for GetArtistSettings (found and not found cases)
+- [x] write tests for SaveArtistSettings (insert and update)
+- [x] write tests for GetAllArtistSettings (empty and populated)
+- [x] run tests — must pass before task 5
 
 ### Task 5: Database layer — CRUD operations for external_releases
 - [ ] implement `GetExternalRelease(db *DB, rgid string) (*ExternalRelease, error)`