Rate limits

429 means slow down. Honor Retry-After and rate-limit headers when present.

Response headers: `RateLimit-Limit`, `RateLimit-Remaining`, `RateLimit-Reset`, and `Retry-After` on 429.

• Mutating /v1 create & batch advertise limits when known
• On 429, clients must honor Retry-After
• Legacy X-RateLimit-* also present