build(image): the image passes a Trivy scan: OS security updates, no runtime pip

The first Trivy scan of the shipped image (0.98.0) found four fixable HIGH
advisories. One was pcre2: a Debian fix newer than the base snapshot, now
taken by `apt-get upgrade` at build time. The others were urllib3 (bumped in
the lock), and msgpack, setuptools and urllib3 vendored inside pip itself.
pip is removed once it has installed the lock, since nothing runs it at
runtime.

A local build of this Dockerfile with --pull scans clean: 0 HIGH/CRITICAL
with a fix, no secrets. .trivyignore.yaml is the publish gate's allowlist,
empty today. Each entry must carry a statement and an expiry date.

.trivyignore.yaml

@@ -0,0 +1,15 @@
+# The CVE gate's allowlist (E32). The publish gate scans the built image and the committed tree with
+# Trivy and fails on any HIGH or CRITICAL finding that has a fix available. A finding that cannot be
+# fixed yet is let through ONLY by an entry here, and every entry carries both:
+#   statement   — why it is acceptable for now (what is exposed, what is not)
+#   expired_at  — the date it stops being accepted; after it, the gate fails again on its own
+# A contract test (frontend/src/lib/gates.contract.test.ts) refuses an entry without either, and
+# docs/gates.md lists them.
+#
+# Example:
+#   vulnerabilities:
+#     - id: CVE-2026-00000
+#       statement: in libfoo's XML path, which nothing in the image calls; no fixed Debian build yet
+#       expired_at: 2026-12-31
+vulnerabilities: []
+secrets: []

Dockerfile

@@ -53,8 +53,11 @@ RUN adduser --disabled-password --gecos "" appuser \
 # Harmless on hosts with no /dev/dri (the driver just goes unused). Deno: the JavaScript runtime yt-dlp
 # uses to solve YouTube's "n" signature challenge for the web player clients (required alongside the PO
 # token, else only storyboards are offered). yt-dlp auto-detects deno on PATH.
+# `apt-get upgrade` (E32): the base image is a snapshot, and the Debian security fixes released since it
+# was cut are only in the archive — the first Trivy scan found one (pcre2) fixed upstream but not in it.
 ARG DENO_VERSION=v2.9.1
 RUN apt-get update \
+    && apt-get upgrade -y --no-install-recommends \
     && apt-get install -y --no-install-recommends ffmpeg intel-media-va-driver unzip curl ca-certificates \
     && curl -fsSL "https://github.com/denoland/deno/releases/download/${DENO_VERSION}/deno-x86_64-unknown-linux-gnu.zip" -o /tmp/deno.zip \
     && unzip -o /tmp/deno.zip -d /usr/local/bin \
@@ -78,9 +81,12 @@ RUN printf 'Types: deb\nURIs: http://deb.debian.org/debian\nSuites: bookworm\nCo
 # Install from the PINNED lock (requirements.lock, generated from requirements.txt) so the image is
 # byte-reproducible and the exact versions are the ones pip-audit scanned in CI — not whatever floats
 # up from the floor ranges on build day. Regenerate the lock when requirements.txt changes (see its header).
+# pip itself is removed once it has installed the lock (E32): nothing runs it at runtime, and it carries
+# its own VENDORED copies of urllib3, msgpack and setuptools, which Trivy reports as the image's own —
+# an attack surface and a CVE stream for a tool the running app never calls.
 COPY backend/requirements.lock .
 RUN --mount=type=cache,target=/root/.cache/pip \
-    pip install --upgrade pip && pip install -r requirements.lock
+    pip install --upgrade pip && pip install -r requirements.lock && pip uninstall -y pip
 
 COPY --chown=appuser backend/ .
 # Read at runtime by config.py as the app_version (a committed single source of truth).