harden: run unprivileged, read-only rootfs, no capabilities

The app is the only process on this box accepting unauthenticated input from
the internet and it was running as root in a writable container. Now uid 10001,
read_only with a tmpfs /tmp, cap_drop ALL and no-new-privileges. Host-side
/srv/scout-website/data and the service account key are chowned to 10001.
Verified on a throwaway container: every route, the admin API, spam rejection,
and a real submission writing to both the jsonl and SQLite.

app/Dockerfile

@@ -2,5 +2,8 @@ FROM python:3.12-slim
 RUN pip install --no-cache-dir fastapi "uvicorn[standard]" python-multipart gspread
 WORKDIR /app
 COPY . /app
-RUN mkdir -p /app/static/img
+RUN mkdir -p /app/static/img && useradd -r -u 10001 app
+# Unprivileged: nothing here needs root, and the process is the one thing on
+# this box that takes unauthenticated input from the internet.
+USER 10001
 CMD ["uvicorn", "app:app", "--host", "0.0.0.0", "--port", "8000"]