P1: Add async completion callback/webhook support

[Neftedollar]

Parent epic: #236

Problem

Async clients must poll repeatedly; no push mechanism on terminal state.

Scope

• Add optional callback registration on start_* (or server-level webhook config).
• Emit signed completion payload on done/failed/cancelled.
• Add retry/backoff and dead-letter behavior.

Acceptance Criteria

• Clients can receive completion events without polling.
• Delivery retries and terminal failure behavior are deterministic.
• Security model documented (signature, replay window, auth token handling).

Non-goals

• Event bus integrations beyond webhook in first iteration.