feat(measurements): surface that the entry timestamp is editable for backdating

The add form already defaults the timestamp to now and accepts any earlier
instant — the plausibility bound and the picker's max both block the future —
but many users never notice they can change it. Add a one-line hint under the
date/time field so logging a past reading is discoverable, and pin the
manual-create contract with a guard asserting the single POST schema strips an
externalId (the stats:-prefix overwrite stays batch-scoped).

src/components/measurements/measurement-form.tsx

@@ -481,6 +481,12 @@ export function MeasurementForm({
           aria-invalid={!!error || undefined}
           aria-describedby={errorDescriptor}
         />
+        {/* v1.17.1 — surface that the timestamp is editable. The picker
+            already defaults to now and accepts any earlier instant; many
+            users never notice they can backdate a reading. */}
+        <p className="text-muted-foreground text-xs">
+          {t("measurements.timestampBackdateHint")}
+        </p>
       </div>
 
       <div className="space-y-2">