[FEATURE] Public API and webhook platform v1

[aernw]

Objective

Expose the first controlled API and webhook surface for external integrations.

Scope

• define the initial public API surface and authentication boundaries
• add webhook subscriptions for key product events where appropriate
• document safe external integration patterns for the first release

Out of Scope

• [Features explicitly not included]
• [Future enhancements to defer]

Acceptance Criteria

• StemHub can integrate into external workflows through defined contracts
• API and webhook claims are supported by a real first version

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M7 - Launch Readiness & Cross-DAW Expansion
Parent workstream(s): #213