P4-1: Create PaymentType enum in core.common

[tcheeric]

Description:
Introduce PaymentType enum for supported payment types.

Changes required:

• Add PaymentType enum to payment-adapter-common.
• Ensure values align with spec (ln, cash, cc, bt).

Acceptance criteria:

• PaymentType enum exists in core.common.
• Enum values map to expected identifiers.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-4-task-1
Dependencies: P3-5