Document god mode (lib/god-mode.ts)

[Railly]

Context

lib/god-mode.ts gates admin routes but is documented nowhere in docs/ or CONTRIBUTING.

Task

Document what god mode grants, who can hold it, and how it is checked; link from CONTRIBUTING.

Acceptance criteria

A contributor touching admin surfaces understands the privilege model from docs alone.

[linear]

HCK0-106

[fxckcode]

Claiming this. Plan: add docs/god-mode.md covering what god mode grants, who holds it (ADMIN_EMAILS allowlist via Clerk primary email), and how it is checked (server-side, currentUser-based), plus link it from CONTRIBUTING. Branch per the repo convention.