Add append-only Nakama ledgers for economy and agent audit

[JOY]

Project: Second Spawn
Area: Nakama
Milestone: Foundation
Priority: P1
Size: M
Review Gate: Local Review
Verification: Backend Tests

Goal

Add append-only audit trails for sensitive systems so snapshots can be debugged, replayed, and investigated.

Scope

• Define ledger records for BodyTime, SECOND, inventory, reward claims, reincarnation, and agent activity.
• Keep current state as snapshots, not as the only source of truth.
• Include actor id, source, reason, amount or delta, target id, validation result, and timestamp where relevant.
• Keep retention bounded for high-volume agent activity.

Acceptance Criteria

• BodyTime and reward mutations write an auditable event.
• Agent decision activity can record model vs fallback source and reason.
• Tests verify ledger writes for at least one economy path and one agent path.