Per-campaign rate limits configurable

Not all campaigns carry the same deliverability risk. A reactivation campaign to a cold list should send at a fraction of the rate appropriate for an active engaged segment. When all campaigns share a single global rate, a high-risk campaign — cold list, broad reach, lower expected engagement — runs at the same throughput as a tight re-engagement campaign, exposing the IP's reputation to the full complaint rate of the worst-performing send. Per-campaign configurable limits let operators deliberately throttle risky campaigns without touching the global settings or pausing the system.

Add rate configuration fields to the campaign schema so the dispatch worker can read per-campaign limits at runtime:

// In prisma/schema.prisma
model Campaign {
  id             String          @id @default(uuid())
  name           String
  subject        String
  maxPerHour     Int?            // null = use system default
  maxDailyVolume Int?            // null = no campaign-level cap
  sendWindow     Json?           // { start: "09:00", end: "17:00", tz: "America/New_York" }
  status         CampaignStatus
  scheduledAt    DateTime?
  createdAt      DateTime @default(now())
}

// In campaign dispatch worker:
export async function getCampaignRateLimit(campaignId: string): Promise<number> {
  const campaign = await db.campaign.findUnique({ where: { id: campaignId } })
  return campaign?.maxPerHour ?? DEFAULT_CAMPAIGN_RATE
}

Read maxPerHour and maxDailyVolume before creating the limiter for each campaign run. Expose these fields in the campaign creation UI so marketing can self-serve conservative limits without an engineering change.