Send Email broadcast

Creates an email broadcast, inserts one row per recipient into `email_messages`, atomically reserves credit (prepaid), and dispatches via AWS SES **synchronously** for `quick broadcast`. The response returns once every recipient has been attempted, with per-recipient send/failed counts. **Required scope:** `send_email` **Rate limit:** 10 requests / 15 min / client ## Billing semantics (prepaid clients) 1. The full `recipientCount × emailRate` is **reserved** at broadcast creation 2. Each **successful** send (HTTP 2xx from SES) is **settled** 3. Each **failed** send (non-2xx or thrown error) **releases** that recipient's portion back to balance A 100-recipient broadcast that succeeds for 95 and fails for 5 ends with 95 charges settled and 5 reserved-credit returned. ## Bounce handling Hard bounces reach your registered webhook URL as a `bounced` event. Suppress these addresses from future broadcasts; SES enforces sender-reputation hygiene.