docs: add admin.canteen.ourpad.casa proxy config to PROJECT.md

NPMPlus proxy ID 20, cert ID 21, backend 192.168.0.127:8901
Cloudflare DNS A record grey cloud, Let's Encrypt SSL

Closes gitea issue #26

PROJECT.md

@@ -28,6 +28,7 @@ canteen-asset-tracker/
 
 - **Port:** 8901 (HTTPS)
 - **URL:** https://canteen.ourpad.casa (NPMPlus reverse proxy → backend :8901)
+- **Admin URL:** https://admin.canteen.ourpad.casa (NPMPlus proxy ID 20, same backend :8901 — admin user login)
 - **Frontend:** Single HTML file — no build step, no npm. All JS/CSS/HTML in one file (~284KB).
 - **Backend:** FastAPI with SQLite (WAL mode). Auth via Bearer tokens. OCR via Tesseract.
 - **Tests:** pytest test suite + Playwright E2E browser tests.