feat(adminsite): image, compose service, image build and docs

Publishes adminsite on 3004 -- 3000 is web, 3003 is the marketing site since
the port shuffle -- and adds the sixth CI image.

CLAUDE.md gains both new services. admin/ was never documented there at all:
the backend plan's wiring task covered compose and CI but not the file every
session actually loads. So this records admin's whole REST surface, its
404-not-403 rule, the three visual identities and, most importantly, that
adminsite/ and site/ share one token set with nothing enforcing the match --
the same hazard shape as sitesvc's mirrored slug rules.

Also notes that admin's REDIS_ADDR reaches admin only, because the base
compose hardcodes redis:6379 for server.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

adminsite/Dockerfile

@@ -0,0 +1,43 @@
+FROM node:26-alpine AS deps
+
+WORKDIR /app
+
+COPY package.json package-lock.json* ./
+RUN npm install
+
+FROM node:26-alpine AS builder
+
+WORKDIR /app
+
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+
+# Baked in at build time and must be reachable from the BROWSER, and present in
+# admin's ADMIN_ORIGIN. Wrong here means every request fails at runtime.
+ARG NEXT_PUBLIC_ADMIN_API_URL=http://localhost:8083
+ENV NEXT_PUBLIC_ADMIN_API_URL=$NEXT_PUBLIC_ADMIN_API_URL
+ARG NEXT_PUBLIC_ADMIN_ENV=production
+ENV NEXT_PUBLIC_ADMIN_ENV=$NEXT_PUBLIC_ADMIN_ENV
+
+RUN npm run build
+
+FROM node:26-alpine AS runner
+
+WORKDIR /app
+
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
+
+RUN addgroup --system --gid 1001 nodejs && \
+    adduser --system --uid 1001 nextjs
+
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+
+USER nextjs
+
+EXPOSE 3000
+ENV PORT=3000
+ENV HOSTNAME="0.0.0.0"
+
+CMD ["node", "server.js"]