Add one person to an active playbook

Call once per trigger (new role, signup, closed deal). The playbook stays on; no policyVersion needed. Read approval in the result:

• waiting with a reason: the person waits in the approval queue; nothing is charged or sent. Hand approvalUrl to a person.
• auto_approved with sendId: the playbook's auto-approve rule (set by a signed-in owner or admin) approved the gift within its per-gift limit and monthly budget and paid it from the balance.
• scheduled: the rule finishes it in the background; watch GET /events for send.auto_approved.
• approved (a person already approved; sendId) or skipped (a person skipped them).

Idempotent on recipient.sourceSystem + recipient.externalId (else recipientId, then a unique email): a repeat returns alreadyQueued:true and never approves or charges twice. A draft or paused playbook refuses with 409 PLAYBOOK_OFF; a batch campaign with 409 NOT_ALWAYS_ON; concurrent adds to one playbook may return 409 PLAYBOOK_BUSY with a short retryAfterMs (send the same call again).