Volume limits documented per ESP and IP

Every ESP plan has hard sending limits — SES sandbox defaults to 1 send per second and 200 per day; SendGrid's free tier caps at 100 sends per second. Exceeding these limits produces 429 errors that fail jobs and degrade IP reputation if retried without backoff. CWE-770 (allocation without limits) applies when queue concurrency is uncapped relative to the ESP tier. The Deliverability Engineering Audit's IP reputation category depends on sends staying within known limits — this check verifies that those limits are actually documented and enforced in code, not just hoped to be safe.

Document ESP limits and wire them to the rate limiter configuration in a dedicated file (e.g., src/config/email-limits.ts):

// src/config/email-limits.ts
export const ESP_LIMITS = {
  sendgrid: { perSecond: 100, perDay: 1_000_000 },
  ses: { perSecond: 14, perDay: 50_000 }  // SES sandbox defaults
} as const

// BullMQ rate limiter:
const limiter = { max: 100, duration: 1000 } // 100/second

Include the source plan tier and date in a comment — limits change when plans change. The queue limiter value must not exceed the documented per-second limit for the configured ESP.