chore: run containers as non-root, add HEALTHCHECK, npm ci

Backend: new `app` user (UID 10001), STORAGE_DIR pre-chowned so the
named volume inherits ownership, curl installed for the HEALTHCHECK
that pings /api/v1/health. The crawler's Chromium uses --no-sandbox
already so dropping privileges costs nothing operationally.

Frontend: switch `npm install` to `npm ci` (matches CI; deterministic
versions; refuses to silently rewrite package-lock.json mid-build).
Run as the built-in `node` user via --chown=node:node, add a busybox
wget HEALTHCHECK on port 3000.

Both images now expose container-level health so orchestrators can
take a wedged container out of rotation instead of letting it keep
serving timeouts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -19,12 +19,49 @@ COPY migrations ./migrations
 RUN touch src/main.rs src/lib.rs && cargo build --locked --release
 
 FROM debian:bookworm-slim
+# `curl` is for the container HEALTHCHECK; `ca-certificates` is for
+# outbound HTTPS (crawler covers/pages).
 RUN apt-get update \
- && apt-get install -y --no-install-recommends ca-certificates \
+ && apt-get install -y --no-install-recommends ca-certificates curl \
  && rm -rf /var/lib/apt/lists/*
+
+# Non-root runtime user. The API binary doesn't need any root
+# privilege; the crawler daemon's Chromium launcher uses --no-sandbox
+# precisely because user-namespace sandboxing is fragile, so dropping
+# privileges costs nothing operationally and shrinks the blast radius
+# of any RCE.
+ARG APP_UID=10001
+ARG APP_GID=10001
+RUN groupadd --system --gid ${APP_GID} app \
+ && useradd --system --uid ${APP_UID} --gid app --home-dir /home/app --create-home --shell /usr/sbin/nologin app
+
 WORKDIR /app
 COPY --from=builder /app/target/release/mangalord /usr/local/bin/mangalord
 COPY --from=builder /app/migrations /app/migrations
+
 ENV STORAGE_DIR=/var/lib/mangalord/storage
+# Pre-create the storage dir so the entrypoint doesn't need to
+# mkdir-as-root and so the named volume mount inherits the right
+# ownership.
+#
+# UPGRADE NOTE for operators: if you're moving from an older image
+# that ran as root, the existing `storage-data` volume has files owned
+# by UID 0 and the new UID-10001 user can't write them. Run once
+# before the upgrade:
+#   docker compose run --rm --user 0 backend \
+#     chown -R 10001:10001 /var/lib/mangalord/storage
+# (Postgres is unaffected — that image's `postgres` user UID hasn't
+# changed.)
+RUN mkdir -p ${STORAGE_DIR} \
+ && chown -R app:app ${STORAGE_DIR} /app /home/app
+
+USER app
 EXPOSE 8080
+
+# `--start-period` is generous because first boot runs sqlx::migrate
+# against postgres which can take a few seconds; subsequent restarts
+# are sub-second.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+  CMD curl -fsS http://localhost:8080/api/v1/health > /dev/null || exit 1
+
 CMD ["mangalord"]