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/database.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"time"
 
@@ -13,6 +14,11 @@ type DB struct {
 	conn *sql.DB
 }
 
+// ErrArtistNotFound is returned by artist lookups when no row matches the given
+// ID. It is a sentinel so callers (e.g. the web UI) can distinguish "missing"
+// from other errors.
+var ErrArtistNotFound = errors.New("artist not found")
+
 // New opens a SQLite database at dbPath and runs schema migrations.
 func New(dbPath string) (*DB, error) {
 	// The _foreign_keys=on DSN parameter enables foreign key enforcement on