chore(docs): add environment variables docs

docs/user/environment.md

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+## Environment variables
+
+Location: project `.env`. Minimal reference for server admins.
+
+### Core
+- HOST: Interface to bind (default `0.0.0.0`)
+- EXPLICIT_FILTER: Filter explicit content (`true|false`, default `false`)
+
+### Redis
+- REDIS_HOST: Hostname (default `redis`)
+- REDIS_PORT: Port (default `6379`)
+- REDIS_DB: Database index (default `0`)
+- REDIS_PASSWORD: Password
+
+### File ownership & permissions
+- PUID: Container user ID owning downloads (default `1000`)
+- PGID: Container group ID owning downloads (default `1000`)
+- UMASK: Default permissions for new files (default `0022`)
+- SKIP_SET_PERMISSIONS: Skip permission fix on startup (`true|false`, default `false`)
+
+### Multi-user & auth
+- ENABLE_AUTH: Enable authentication (`true|false`, default `false`)
+- JWT_SECRET: Long random string for tokens (required if auth enabled)
+- JWT_EXPIRATION_HOURS: Session duration in hours (default `720`)
+- DEFAULT_ADMIN_USERNAME: Seed admin username (default `admin`)
+- DEFAULT_ADMIN_PASSWORD: Seed admin password (change it!)
+- DISABLE_REGISTRATION: Disable public signups (`true|false`, default `false`)
+
+### SSO
+- SSO_ENABLED: Enable SSO (`true|false`)
+- SSO_BASE_REDIRECT_URI: Base backend callback (e.g. `http://127.0.0.1:7171/api/auth/sso/callback`)
+- FRONTEND_URL: Public UI base (e.g. `http://127.0.0.1:7171`)
+- GOOGLE_CLIENT_ID / GOOGLE_CLIENT_SECRET
+- GITHUB_CLIENT_ID / GITHUB_CLIENT_SECRET
+
+Tips:
+- If running behind a reverse proxy, set `FRONTEND_URL` and `SSO_BASE_REDIRECT_URI` to public URLs.
+- Change `DEFAULT_ADMIN_*` on first login or disable registration and create users from the admin panel.