P4-2: Create GatewayRegistry

[tcheeric]

Description:
Add registry to discover gateways by payment type.

Changes required:

• Implement GatewayRegistry with Spring auto-discovery.
• Provide lookup by PaymentType and gatewayId.

Acceptance criteria:

• GatewayRegistry resolves gateways by type and ID.
• Registry is wired in application context.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-4-task-2
Dependencies: P4-1