P3-4: Replace WebhookServlet with WebhookDispatcherServlet

[tcheeric]

Description:
Replace legacy servlet with dynamic dispatcher servlet.

Changes required:

• Implement WebhookDispatcherServlet for /webhook/{paymentType}.
• Keep /webhook/phoenixd as alias during transition.

Acceptance criteria:

• Dispatcher servlet routes based on paymentType.
• Legacy endpoint remains available as alias.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-3-task-4
Dependencies: P3-2