fix(scanner): ensure full scans update the DB (#4252)

* fix: ensure full scan refreshes all artist stats

After PR #4059, full scans were not forcing a refresh of all artists.
This change ensures that during full scans, all artist stats are refreshed
instead of only those with recently updated media files.

Changes:
- Set changesDetected=true at start of full scans to ensure maintenance operations run
- Add allArtists parameter to RefreshStats() method
- Pass fullScan state to RefreshStats to control refresh scope
- Update mock repository to match new interface

Fixes #4246
Related to PR #4059

* fix: add tests for full and incremental scans

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

tests/mock_artist_repo.go

@@ -94,4 +94,18 @@ func (m *MockArtistRepo) UpdateExternalInfo(artist *model.Artist) error {
 	return nil
 }
 
+func (m *MockArtistRepo) RefreshStats(allArtists bool) (int64, error) {
+	if m.Err {
+		return 0, errors.New("mock repo error")
+	}
+	return int64(len(m.Data)), nil
+}
+
+func (m *MockArtistRepo) RefreshPlayCounts() (int64, error) {
+	if m.Err {
+		return 0, errors.New("mock repo error")
+	}
+	return int64(len(m.Data)), nil
+}
+
 var _ model.ArtistRepository = (*MockArtistRepo)(nil)