feat(auth): sessions expire server-side; password change and reset end other sessions (M462 #4978)

Sessions had no server-side expiry: only the web cookie's 30-day Max-Age
limited them, and a bearer token (Android) lived until revoked by hand.
GetSessionByTokenHash and ListSessionsForUser now ignore sessions idle for
30 days or older than a year, and the GC worker deletes them hourly.

A password change was a plain UPDATE, so a session opened with the old
password survived it. Now:
- self-service change signs out every other device and keeps this one;
- reset by email ends every session the account has;
- an admin reset ends the target's sessions (keeping the admin's own when
  they reset themselves).

The success copy on web and Android says the other devices were signed out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/dbq/gc.sql.go

@@ -84,6 +84,22 @@ func (q *Queries) GcDeleteExpiredPasswordResets(ctx context.Context) (int64, err
 	return result.RowsAffected(), nil
 }
 
+const gcDeleteExpiredSessions = `-- name: GcDeleteExpiredSessions :execrows
+DELETE FROM sessions
+WHERE last_seen_at <= now() - interval '30 days'
+   OR created_at <= now() - interval '365 days'
+`
+
+// Sessions past their idle (30 days) or absolute (1 year) limit. They already
+// fail auth through GetSessionByTokenHash's filter; this only clears the rows.
+func (q *Queries) GcDeleteExpiredSessions(ctx context.Context) (int64, error) {
+	result, err := q.db.Exec(ctx, gcDeleteExpiredSessions)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
 const gcExpireScrobbleQueueFailedRows = `-- name: GcExpireScrobbleQueueFailedRows :execrows
 DELETE FROM scrobble_queue
  WHERE status = 'failed'