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 @@
 # Stage 1: Build
 FROM node:20-alpine AS build
-
+RUN apk update && apk upgrade --no-cache
 WORKDIR /app
 
 COPY package.json package-lock.json ./
@@ -11,6 +11,9 @@ RUN npm run build
 
 # Stage 2: Production — uses nginxinc/nginx-unprivileged which runs as non-root (UID 101)
 FROM nginxinc/nginx-unprivileged:stable-alpine AS prod
+USER root
+RUN apk update && apk upgrade --no-cache
+USER 101
 
 COPY --from=build /app/dist /usr/share/nginx/html
 COPY nginx.conf /etc/nginx/conf.d/default.conf