Add STAGING — NOT FOR PRODUCTION banner

fix(staging): mirror prod /app/* absolute layout (KILLINCHU_ROOT=/app, static, drones_db) — Doctrine v11

fix(staging): insert env-aware /healthz before SPA catch-all; mirror prod /app layout — Doctrine v11

feat(staging): killinchu staging Space — same source as prod, SZL_ENV=staging — Doctrine v11 749/14/163 — Yachay

initial commit