fix(api): give the shared write ceiling a code a client can branch on

The eleven single-record writes answered their shared 429 with prose and
nothing else, while the envelope's own description tells a client to
branch on `meta.errorCode`. On `POST /api/medications/[id]/intake` and
`POST /api/cycle/day-logs` a second, narrower bucket can answer 429 as
well, so the presence of the rate-limit headers was the only thing
telling the two refusals apart.

All eleven now carry `record_write.rate_limited`, the catalogue lists
it, the published 429 names it, and the contract test fails if a site
stops emitting it or the sentence stops publishing it.

The same description now states what a client could not predict from
"300 requests per 60 seconds": refused requests count too, because the
check runs before the body is read.

src/app/api/cycle/day-logs/route.ts

@@ -105,7 +105,9 @@ async function postDayLog(request: NextRequest): Promise<Response> {
   // client hits were not.
   const writeRl = await checkRecordWriteRateLimit(actor.id);
   if (!writeRl.allowed) {
-    return apiError("Too many writes, try again later", 429);
+    return apiError("Too many writes, try again later", 429, {
+      errorCode: "record_write.rate_limited",
+    });
   }
 
   const { data: rawBody, error: jsonError } = await safeJson(request, {