Implement audit log table and service`

[Nemenwq]

Dependencies: Issue #27, Issue #28.

Context:
Sensitive actions must be recorded for security and compliance.

Tasks:

• Create an audit_logs table/entity with fields:
  • id, merchant_id (nullable), actor_type, actor_id (nullable), action, target_type, target_id, metadata, created_at
• Add a lightweight audit log service with helper methods.
• Log at least:
  • API key create/revoke
  • webhook create/delete
  • withdrawal request

Implementation Notes:

• Don’t log secrets (API keys, webhook secrets) — log identifiers and metadata only.

Scope:

• In scope: DB table + service + usage in endpoints.
• Out of scope: Admin UI.

Files: backend/api_gateway/src/audit/