Receive a LegiScan push payload, store it durably, and ack

Authentication via Authorization: Token <key>, X-API-Key header, or ?key= query parameter.

Payload types: bill, session, people, text, vote, amendment, supplement. Stores the raw payload as a pending queue row, answers the missing question from indexed reads, and returns. The serialized drain worker (services/legiscan_push_queue.py) does the row mutation, because processing inline ran 5.7-27.8s past LegiScan's 20s deadline and deadlocked against the second source IP (FED-1000).