fix: persist and replay original status code in idempotency layer

[tmchow]

Bug

The idempotency layer had a status_code column in the DB and an options.statusCode parameter in storeIdempotencyResult(), but the wiring was never completed:

Location	Problem
storeIdempotentMutationResult()	Called storeIdempotencyResult without passing statusCode
getStoredIdempotencyRecord()	Never read status_code from the DB
maybeReplayIdempotentMutation()	Hardcoded 200 in the replay response

Result: every idempotency replay returned 200 regardless of the original status — e.g. a 201 Created became 200 OK on retry.

Fix

Threaded status_code through all three layers in server/db.ts and server/agent-routes.ts:

// storeIdempotentMutationResult — pass status through
storeIdempotencyResult(slug, routeKey, replay.idempotencyKey, body, replay.requestHash, { statusCode: status })

// getStoredIdempotencyRecord — read it back
SELECT response_json, request_hash, status_code FROM mutation_idempotency ...
return { response, requestHash, statusCode }

// maybeReplayIdempotentMutation — use stored value, fallback to 200 for legacy rows
sendMutationResponse(res, stored.statusCode ?? 200, stored.response)

Legacy rows without status_code fall back to 200 (the column defaults to 200 per the existing migration).