chore(security): remove committed secrets/dumps, .env-based config, hardened deploy webhook

- Delete backups/*.dump and all real credentials from README/docs
  (prod MinIO creds, Casdoor passwords, server IP, admin password)
- All stack secrets now come from git-ignored .env (.env.example added);
  docker-compose.prod.yml refuses to boot without them (${VAR:?})
- seed_casdoor.py: bcrypt password hashing, generated console/client secrets
- Harden infra/deploy-webhook: HMAC-verified Gitea webhook receiver
  + systemd unit + README (replaces unauthenticated deploy endpoint)
- docs/SECRET-MANAGEMENT.md: secret inventory + rotation checklist

frontend/Dockerfile

@@ -13,9 +13,10 @@ ARG NPM_REGISTRY=https://registry.npmmirror.com
 COPY package.json package-lock.json ./
 RUN npm config set registry ${NPM_REGISTRY} && npm ci
 
-# Developer Mode panel on the login page (default enabled; set to
-# "false" to build a production login without the dev panel).
-ARG DEV_LOGIN_ENABLED=true
+# Developer Mode panel on the login page (dev-only aid). Production builds
+# MUST keep this "false" — identity comes from Casdoor tokens, and the
+# backend rejects the X-User-* header path unless DEV_LOGIN_ENABLED=true.
+ARG DEV_LOGIN_ENABLED=false
 ENV VITE_ENABLE_DEV_LOGIN=${DEV_LOGIN_ENABLED}
 
 # Source code + build