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/public/favicon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#3B82F6;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#6366F1;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  <rect width="100" height="100" rx="20" fill="url(#grad)"/>
+  <text x="50" y="65" text-anchor="middle" fill="white" font-family="Inter, sans-serif" font-weight="700" font-size="40">IH</text>
+</svg>