feat: add graceful shutdown endpoint for UVM API server and update documentation

USAGE.md

@@ -141,6 +141,21 @@ go to stderr. VM JSON omits passwords. Lifecycle and sync results are arrays of
 `changes`). `ports list --json` returns an array even for one VM. JSON does
 not disable confirmations, and errors may still be plain text on stderr.
 
+### Stop the Management API
+
+The API can be stopped remotely through its authenticated shutdown endpoint.
+This stops only the UVM API process; it does not stop or delete any VM:
+
+```sh
+curl -X POST \
+  -H "X-UVM-Token: $UVM_API_TOKEN" \
+  http://127.0.0.1:8000/exit
+```
+
+The server first returns `202 {"status":"stopping"}`, then performs Uvicorn's
+graceful shutdown. Use the actual API host and port when they differ from the
+defaults. `GET /exit` is intentionally not supported.
+
 ## 4. Install UVM Assets
 
 UVM installs host packages, Firecracker, Jailer, a guest kernel, and a guest