docs(timeline): flag DatePrecision as a hand-maintained backend mirror

Note above the DatePrecision type that it mirrors the Java DatePrecision enum,
must be updated manually in lockstep with that enum, and must not be migrated
to the OpenAPI-generated type — it drives the shared client-side formatter
shared by documents and the timeline date-label facade.

Refs #778
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/shared/utils/documentDate.ts

@@ -4,6 +4,13 @@ import { m } from '$lib/paraglide/messages.js';
 /**
  * Precision of a document's date — mirrors the backend {@code DatePrecision} enum
  * and the import normalizer's seven values verbatim.
+ *
+ * DRIFT RISK: this is a hand-maintained mirror of the Java {@code DatePrecision}
+ * enum, NOT an OpenAPI-generated type. It must be updated manually whenever the
+ * Java enum changes, and must NOT be migrated to the generated API type — the
+ * generated enum is request/response-shaped, while this drives the shared
+ * client-side formatter (used by both documents and the timeline façade). Keep
+ * the two in lockstep by hand.
  */
 export type DatePrecision = 'DAY' | 'MONTH' | 'SEASON' | 'YEAR' | 'RANGE' | 'APPROX' | 'UNKNOWN';