P3-3: Create payment-adapter-ln-webhook with PhoenixWebhookHandler

[tcheeric]

Description:
Introduce LN webhook module and implement Phoenix webhook handler.

Changes required:

• Create payment-adapter-ln-webhook module.
• Implement PhoenixWebhookHandler using the new SPI.

Acceptance criteria:

• LN webhook module builds and registers handler.
• Phoenix webhook logic is routed through dispatcher.

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