fix(timeline): harden curator event precision field

- Validate the submitted precision against the DatePrecision allow-list in
  parseEventForm (falls back to DAY) so an untrusted token can't flow into
  the request body — symmetric with the existing `type` narrowing.
- Parameterize the precision input name via DatePrecisionField's new
  precisionInputName prop; the timeline form now submits `precision` instead
  of the misleading document-domain `metaDatePrecision`. Document form keeps
  the default, so its behaviour is unchanged.
- Capture EventTypeSelect's onchange into EventForm's `type` state so it no
  longer goes stale (the submitted value was already correct via the hidden
  input; this keeps the local state in sync).

Addresses PR #832 review (#781).

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

frontend/src/lib/shared/primitives/DatePrecisionField.svelte

@@ -16,7 +16,9 @@ import type { DatePrecision } from '$lib/shared/utils/documentDate';
  * Exposed (shared contract — both WhoWhenSection and EventForm depend on it):
  *   - dateIso, precision, endDateIso  — $bindable; the parent's binding IS the
  *     state (no redundant $state mirror).
- *   - dateInputName / endDateInputName — names of the hidden ISO inputs.
+ *   - dateInputName / endDateInputName / precisionInputName — submitted field
+ *     names; defaults match the document form (`metaDatePrecision`), the timeline
+ *     form overrides precisionInputName to `precision`.
  *   - initialDateIso / suggestedDateIso — seeding inputs (see onMount + $effect).
  *   - dateTestId / precisionTestId / endDateInnerTestId — forwarded data-testid
  *     attributes so existing WhoWhenSection selectors survive the extraction.
@@ -28,6 +30,7 @@ let {
 	endDateIso = $bindable(''),
 	dateInputName = 'documentDate',
 	endDateInputName = 'metaDateEnd',
+	precisionInputName = 'metaDatePrecision',
 	initialDateIso = '',
 	suggestedDateIso = '',
 	dateLabel = m.form_label_date(),
@@ -41,6 +44,7 @@ let {
 	endDateIso?: string;
 	dateInputName?: string;
 	endDateInputName?: string;
+	precisionInputName?: string;
 	initialDateIso?: string;
 	suggestedDateIso?: string;
 	dateLabel?: string;
@@ -145,7 +149,7 @@ $effect(() => {
 	</label>
 	<select
 		id="{dateInputName}Precision"
-		name="metaDatePrecision"
+		name={precisionInputName}
 		bind:value={precision}
 		class="block min-h-[48px] w-full rounded border border-line px-2 py-3 text-sm shadow-sm focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 	>