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.

app/middleware/versioning.py

@@ -0,0 +1,39 @@
+"""
+Middleware de versionnement d'API.
+
+Ajoute l'en-tête `X-API-Version` aux réponses des routes sous `/api/v1/`
+et gère un éventuel avertissement de dépréciation via Sunset.
+"""
+import logging
+from typing import Awaitable, Callable
+
+from fastapi import Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware
+
+from app.config import settings
+
+logger = logging.getLogger(__name__)
+
+
+class APIVersioningMiddleware(BaseHTTPMiddleware):
+    async def dispatch(
+        self, request: Request, call_next: Callable[[Request], Awaitable[Response]]
+    ) -> Response:
+        response = await call_next(request)
+
+        # Appliquer X-API-Version seulement aux routes versionnées (ex: /api/v1/)
+        path = request.url.path
+        if path.startswith("/api/v1/"):
+            response.headers["X-API-Version"] = "v1"
+
+            # Si une date de fin de support est configurée, l'ajouter en en-tête
+            if settings.API_V1_SUNSET_DATE:
+                response.headers["Sunset"] = settings.API_V1_SUNSET_DATE
+                # Logger un avertissement si on l'utilise
+                logger.warning("api.v1_sunset_warning", extra={
+                    "client": request.client.host if request.client else "unknown",
+                    "path": path,
+                    "sunset": settings.API_V1_SUNSET_DATE
+                })
+
+        return response