feat(app): Next.js 16 + Auth.js v5 portal skeleton

Next.js 16 + Auth.js v5 skeleton: host→slug middleware, tenant-context layout, OIDC sign-in flow against breakpilot-dev realm. 100% coverage on src/lib. Bumps next to 16.2.6 to clear trivy CVEs in 15.0.3.

Dockerfile

@@ -0,0 +1,25 @@
+# Multi-stage Next.js build using output: 'standalone'.
+
+FROM node:20-alpine AS deps
+WORKDIR /app
+RUN corepack enable && corepack prepare pnpm@9 --activate
+COPY package.json pnpm-lock.yaml ./
+RUN pnpm install --frozen-lockfile
+
+FROM node:20-alpine AS build
+WORKDIR /app
+RUN corepack enable && corepack prepare pnpm@9 --activate
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+RUN pnpm build
+
+FROM node:20-alpine AS run
+WORKDIR /app
+ENV NODE_ENV=production
+ENV PORT=3000
+COPY --from=build /app/public ./public
+COPY --from=build /app/.next/standalone ./
+COPY --from=build /app/.next/static ./.next/static
+USER node
+EXPOSE 3000
+CMD ["node", "server.js"]