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/web/server.go

@@ -40,6 +40,11 @@ func NewServer(cfg *config.ServerConfig, db *database.DB, uiBaseURL string) *Ser
 	}
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", s.handleDashboard)
+	mux.HandleFunc("/artist/{id}", s.handleArtist)
+	mux.HandleFunc("/artist/{id}/ignore", s.ignoreOrRestore)
+	mux.HandleFunc("/artist/{id}/restore", s.ignoreOrRestore)
+	mux.HandleFunc("/artist/{id}/ignore-singles", s.toggleIgnoreSingles)
+	mux.HandleFunc("/archive", s.handleArchive)
 	s.mux = mux
 	return s
 }