feat: add edit artist endpoint (section 1.4)

- Add GET/PUT /api/artists/{id} for artist settings
- Update sync to create artists table entry (library settings)
- Support partial updates for monitored, path, quality/metadata profiles
- Add e2e tests for get, edit, partial update flows

internal/services/download.go

@@ -101,6 +101,10 @@ func Sync(
 			idStr := id.String()
 			artistMetadataID = &idStr
 			log.Info().Str("artist", artist.Name).Str("id", idStr).Msg("stored artist metadata")
+
+			if _, err := db.UpsertArtist(ctx, id); err != nil {
+				log.Warn().Err(err).Str("artist", artist.Name).Msg("failed to create artist library entry")
+			}
 		}
 	}
 
@@ -306,7 +310,7 @@ func RefreshArtist(
 		return nil, &NotFoundError{Message: "database not available"}
 	}
 
-	existingArtist, err := db.GetArtistByForeignID(ctx, foreignArtistID)
+	existingArtist, err := db.GetArtistMetadataByForeignID(ctx, foreignArtistID)
 	if err != nil {
 		return nil, &NotFoundError{Message: "artist not found: " + foreignArtistID}
 	}