feat(providers): add System One (Jev) decision endpoint

New /v1/systemone pass-through route for Jev decision models (jev-1.13,
jev-1.13-free) on OpenCode Zen and the free lane. Follows the media-route
pattern: systemoneConfig in the registry drives URL/headers, the handler
mirrors the embeddings account-fallback + usage flow, and the dashboard
gains a System One media-provider kind. No chat-pipeline changes.

Co-Authored-By: Claude Code <noreply@anthropic.com>

next.config.mjs

@@ -75,6 +75,10 @@ const nextConfig = {
         source: "/responses",
         destination: "/api/v1/responses"
       },
+      {
+        source: "/systemone",
+        destination: "/api/v1/systemone"
+      },
       {
         source: "/v1beta/:path*",
         destination: "/api/v1beta/:path*"