Post Ar Scheduler Send Invoice

Manual send -- v1 parity: POST /send-invoice (ar-agent.ts:999-1145). Always sends the first invoice step; the automated cadence owns every later step. Goes through the SAME claim-once floor, template resolution, CC/BCC and PDF-attachment path as the automated sender. Does NOT send a still-draft invoice (a deliberate, pre-existing v2 gate this build honors rather than loosens) -- see app/modules/finance/ar_scheduler_send.py send_manual_invoice's own docstring for the exact v1 divergence.