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>

src/app/api/v1/systemone/route.js

@@ -0,0 +1,21 @@
+import { handleSystemone } from "@/sse/handlers/systemone.js";
+
+/**
+ * Handle CORS preflight
+ */
+export async function OPTIONS() {
+  return new Response(null, {
+    headers: {
+      "Access-Control-Allow-Origin": "*",
+      "Access-Control-Allow-Methods": "POST, OPTIONS",
+      "Access-Control-Allow-Headers": "*"
+    }
+  });
+}
+
+/**
+ * POST /v1/systemone - System One (Jev) decision endpoint
+ */
+export async function POST(request) {
+  return await handleSystemone(request);
+}