feat: Async sync_creatives lifecycle (submitted/working/input-required responses)

[KonstantinMirin]

Summary

AdCP spec defines 3 async response schemas for sync_creatives:

• SyncCreativesAsyncResponseSubmitted — returned when sync operation is queued
• SyncCreativesAsyncResponseWorking — progress info during processing
• SyncCreativesAsyncResponseInputRequired — pause conditions requiring human input

Library types exist in adcp.types.generated_poc.media_buy. Salesagent has zero implementation.

Spec Obligations

• UC-006-ASYNC-LIFECYCLE-01 (P3, MUST)
• UC-006-ASYNC-LIFECYCLE-02 (P3, MUST)
• UC-006-ASYNC-LIFECYCLE-03 (P3, MUST)

Implementation Scope

1. Extend schemas with local types inheriting from library async response types
2. Add async mode parameter to sync_creatives
3. Background task queue (Celery/RQ) for async creative processing
4. Progress tracking during sync
5. Pause/resume mechanism for input-required scenarios
6. Status endpoint for checking async task progress

Context

This aligns with the broader architectural direction: adapters should NOT run in the HTTP request cycle. The correct flow is accept order → validate against local Postgres → return 201 pending → background worker calls adapter → update status → notify buyer.

Currently 3 xfailed tests cover these obligations in tests/integration/test_creative_sync_behavioral.py.