Add admin panel, WebSocket support, and API versioning

Introduce an admin portal (React + Nginx), WebSocket routing, and
API versioning middleware with `/api/v1/` prefix deprecation.
Add master API key authentication, new Prometheus metrics for AI
token consumption and active WebSockets, and extend S3 config
with a public endpoint URL. Update test paths and fixtures to
align with the new routing structure.

imago-admin/Dockerfile

@@ -0,0 +1,16 @@
+# Stage 1 : Build
+FROM node:20-alpine AS builder
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+ARG VITE_API_BASE_URL=""
+ARG VITE_APP_TITLE="Imago Hub Admin"
+RUN npm run build
+
+# Stage 2 : Serve
+FROM nginx:alpine
+COPY --from=builder /app/dist /usr/share/nginx/html
+COPY nginx/default.conf /etc/nginx/conf.d/default.conf
+EXPOSE 3000
+CMD ["nginx", "-g", "daemon off;"]