Add an email recipient to a schedule.

Adds an email recipient to the schedule (ADR-0051 §3). Requires dashboard:write. The email is NFC-normalized and lowercased before classification:

• If the email matches a current org member → status=active (immediate delivery).
• If external (not a member) → status=pending_approval. No email is sent to pending or rejected recipients — approval must be explicit (approve endpoint). email and is_external are immutable after insert; to change an address, delete and re-add the recipient (which re-enters the approval state machine). Returns 409 if the email is already added to this schedule.