P3-2: Implement WebhookHandler SPI, Registry, Dispatcher

[tcheeric]

Description:
Add SPI interfaces and core dispatcher/registry for webhook handling.

Changes required:

• Implement WebhookHandler and WebhookPayload interfaces.
• Add WebhookRegistry and WebhookDispatcher components.

Acceptance criteria:

• Webhook SPI interfaces exist and are wired by Spring.
• Dispatcher routes to registry-registered handlers.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-3-task-2
Dependencies: P3-1