feat(labs): refuse a biomarker unit change while readings are linked

Readers label every linked reading with the marker's unit, so changing
it relabelled stored numbers without converting them. PUT
/api/biomarkers/{id} now answers 422 biomarkers.unit.locked with
meta.readingCount when the unit differs and any reading (tombstoned
ones included) is linked. The marker editor says so under the unit
field.

src/lib/openapi/error-codes.ts

@@ -76,6 +76,7 @@ export const ERROR_CODE_CATALOGUE: Readonly<Record<string, readonly string[]>> =
       "backup.section.missing",
       "backup.upload.too_large",
     ],
+    biomarkers: ["biomarkers.unit.locked"],
     "body-site": ["body-site.invalid"],
     coach: [
       "coach.budget.exceeded",