chore: promote uat to production (Grype image vulnerability scanning)

Merges Grype-based container image vulnerability scanning and Docker CVE remediation to production.

- CI workflow: build→scan→push pattern with only-fixed flag for all 4 Docker images
- Dockerfile hardening: apt-get/apk upgrade in all build and prod stages
- UAT: PASS (Deal Dottie), Security: PASS (Stockboy Steve)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

Dockerfile

@@ -1,6 +1,6 @@
 FROM python:3.12-slim AS build
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+RUN apt-get update && apt-get upgrade -y && apt-get install -y --no-install-recommends \
     libpq-dev \
     build-essential \
     && rm -rf /var/lib/apt/lists/*
@@ -12,7 +12,7 @@ RUN pip install --no-cache-dir --prefix=/install .
 
 FROM python:3.12-slim AS prod
 
-RUN apt-get update && apt-get install -y --no-install-recommends libpq5 && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get upgrade -y && apt-get install -y --no-install-recommends libpq5 && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
 RUN adduser --system --group --uid 1000 app