Get Ar Scheduler Activity

The Email Audit Log tab -- v1 parity: GET /activity (ar-agent.ts:966-995), read from send_receipts (v2's own claim-once floor, app/platform/email_out.py) instead of v1's activity_log table: every real AR send already writes a 'sent' row there, keyed ar:{invoice_id}:{cadence_step} (ar_sender.ar_send_idempotency_key), so this is the same durable record the send pipeline itself relies on, not a second bookkeeping table that could drift out of sync with it. Test sends (POST .../test below) deliberately bypass send_receipts entirely (see that route's own docstring) so they never appear here.