fix(docker): refuse a root or non-numeric LOCAL_USER_ID

LOCAL_USER_ID=0 passed every check and reached `setpriv --reuid=0`: the bot would run root while looking properly configured, and useradd -o accepted the duplicate uid without complaint. A non-numeric value failed later inside useradd behind `|| true`, which hid the real cause. The entrypoint now rejects both up front with a message naming the variable, and the id/uid expansions are quoted so a value with spaces cannot word-split into extra useradd arguments (audit SEC-007).

Verified by sourcing the entrypoint under a faked `id`: uid 0, 'abc' and '12 3' each exit 1 with the refusal line, while a non-root caller passes straight through to exec. AGENTS and .env.example state the constraint.

AGENTS.md

@@ -87,7 +87,7 @@ Docker: `docker build -t tgxmb .` then `docker run --rm -d --name tgxmb --env-fi
 | `crates/x-media/src/site/{mod,download}.rs` | `mod.rs`: dispatcher, `Fetched`/`FetchError`, `needs_media_headers` (the per-site rule, asked by the inline path to skip what Telegram cannot fetch). `download.rs`: the media-download stack — the metadata vs. media HTTP clients, the host-network guard (applied to the start URL and every redirect hop) and `download_media_limited`/`download_media_to_file` (which add the site's headers, e.g. `Referer: https://www.pixiv.net/` for `pximg.net`) |
 | `crates/x-media/src/site/pixiv/api.rs` | OAuth token exchange (hardcoded app client id/secret), access-token cache, ugoira zip→MP4 via ffmpeg in `spawn_blocking` |
 | `Dockerfile` | Multi-stage: cached dep layer via stub sources + `touch *.rs` mtime bump (cargo's freshness is mtime-based and `cargo clean -p` removes 0 files — the touch is what forces the real sources to rebuild while deps stay cached), static ffmpeg from ffmpeg.martin-riedl.de (`FFMPEG_URL` arg, optional `FFMPEG_SHA256` checksum, `unzip -t` integrity check), `debian:bookworm-slim` runtime, entrypoint. Runtime ships **no libssl/libcrypto/CA bundle** — rustls webpki-roots handles all TLS, and the static ffmpeg only processes local files (downloads go through reqwest) |
-| `docker-entrypoint.sh` | Privilege drop: `useradd` with `LOCAL_USER_ID` (default 9001) + `setpriv` (no gosu on bookworm-slim) |
+|`docker-entrypoint.sh` | Privilege drop: `useradd` with `LOCAL_USER_ID` (default 9001) + `setpriv` (no gosu on bookworm-slim); rejects a non-numeric or `0` `LOCAL_USER_ID`, which would otherwise survive the drop and run the bot root |
 | `docker-compose.yml` | The deployment composition, committed as-is: every instance value (token, admins, site credentials, domain) is a `${VAR}` substitution read from the gitignored `.env` beside it, so the file needs no per-deployment edit — and a variable not listed in a service's `environment:` never reaches that container. Ships nginx-proxy + acme-companion: webhook mode needs TLS termination in front (teloxide's axum listener is HTTP-only; `WEBHOOK_CERT` only feeds `set_webhook`), bot exposes `VIRTUAL_HOST`/`VIRTUAL_PORT` on the shared `proxy` network, no host port; container names `nginx-proxy`/`acme-companion`/`tgxmb`, start order via `depends_on` (proxy → acme → bot) |
 | `.github/workflows/docker.yml` | CI: build+push to Docker Hub on tag `v*`/master, plus a build-only check on PRs touching the build inputs; **no test step**; verifies a release tag matches both crate versions; buildx gha cache (`cache-from` always, `cache-to` except on PRs, scope `tgxmb-build`, `mode=max`) so cargo deps + ffmpeg layers are restored across runs; `FFMPEG_URL`/`FFMPEG_SHA256` come from repo variables when set |
 | `README.md` | Feature docs + command table (Chinese) |