docs(v2): add top-level README with architecture overview

Elliot Forbesrequested to merge
ef-v2-readme into master

What

Adds v2/README.md — a top-level overview of the v2 module.

Why

The v2 module had no entry-point document. A new engineer's first question is "which of these packages do I need and how do they fit together?" — that required reading 8 separate package docs to answer. This README answers it in one place.

What's covered

  • Package index — one-line description of every package in the module
  • Typical service wiring — a complete main() showing app + postgres + httpserver wired together with readiness checks
  • Dependency diagram — ASCII diagram showing what provides what and what plugs into what
  • Observability defaults — the three environment variables (GITLAB_LOG_LEVEL, GITLAB_LOG_FORMAT, GITLAB_TRACING) that control behaviour out of the box
  • Testing pattern — how apptest / logtest / tracetest fit together for unit tests with no live infrastructure
  • Module path — clear statement of the v2 import path and the v1/v2 distinction

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