Add activity domain module scaffold for normalized event history and timeline queries

[David-patrick-chuks]

Summary

Add an activity module scaffold for normalized event history and future timeline-oriented queries.

Context

A finance and agent orchestration platform will need a unified way to represent activity across agents, payments, wallets, and infrastructure events.

Scope

• Create an activity module scaffold
• Define normalized activity/event record concepts
• Prepare service boundaries for future timeline queries
• Keep the implementation scaffolded and issue-ready

Acceptance Criteria

• An activity module exists with clear domain boundaries
• Event history concepts are modeled consistently
• The structure is suitable for future timeline and feed endpoints
• Contributors can extend the module without cross-cutting refactors

Out of Scope

• Full feed UI concerns
• Event ingestion from all future systems
• Analytics dashboards

Dependencies

Pairs well with #14, #18, and #25.
Request tracing, audit metadata, and webhook/event-oriented work make this module more grounded.