feat: add netplan configuration models and backend API for applying netplan

backend/src/main.py

@@ -1,3 +1,6 @@
+from asyncio import subprocess
+from http.client import HTTPException
+import src.Models.netplan as netplan
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 import os
@@ -25,4 +28,33 @@ def read_root():
 @app.get("/api/nft/ruleset")
 def read_root():
     message = os.popen("sudo nft --json list ruleset").read().strip()
-    return {"message": message}
+    return {"message": message}
+
+
+NETPLAN_FILE = "/etc/netplan/99-app.yaml"
+
+@app.post("/netplan/apply")
+def apply_netplan(config: netplan.NetworkConfig):
+    try:
+        data = {"network": config.dict(exclude_none=True)}
+
+        with open(NETPLAN_FILE, "w") as f:
+            yaml.dump(data, f, sort_keys=False)
+
+        # Safety: rollback if config breaks connectivity
+        result = subprocess.run(
+            ["netplan", "try", "--timeout", "10"],
+            capture_output=True,
+            text=True,
+        )
+
+        if result.returncode != 0:
+            raise HTTPException(
+                status_code=500,
+                detail=f"Netplan failed: {result.stderr}"
+            )
+
+        return {"status": "ok", "output": result.stdout}
+
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=str(e))