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.

docker-compose.yml

@@ -1,4 +1,23 @@
 services:
+  # ── Portail Admin (React + Nginx) ──────────────────────────
+  admin:
+    build:
+      context: ./imago-admin
+      args:
+        VITE_API_BASE_URL: ""
+    ports:
+      - "3000:3000"
+    depends_on:
+      backend:
+        condition: service_healthy
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:3000"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+
+  # ── Backend Imago Hub (FastAPI + Uvicorn) ──────────────────
   backend:
     build: .
     ports: