feat: add Web UI artist detail, archive, and ignore actions

Implements Task 8: artist detail page (local albums + found-missing
with ignore buttons), ignored-releases archive with restore, and POST
handlers toggling ignore flags and ignore_singles. Adds ErrArtistNotFound
sentinel so callers can distinguish missing artists, and wires routes via
the enhanced ServeMux path wildcard.

internal/database/artist_settings.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 )
 
@@ -17,6 +18,9 @@ func GetArtistSettings(db *DB, id string) (*ArtistSettings, error) {
 		id,
 	).Scan(&s.ID, &s.Name, &mbid, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored)
 	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, ErrArtistNotFound
+		}
 		return nil, err
 	}
 	s.MBID = mbid.String