feat: add Python install script for on-demand AI feature bundles

Reads the feature manifest, installs pip packages (common + arch-specific),
downloads models in parallel with atomic rename, and writes installed.json.
Includes disk space pre-check, NCCL conflict handling, retry logic, and
progress reporting via stderr JSON lines. Also updates the feature route
to pass manifestPath and modelsDir as CLI arguments.

apps/api/src/routes/features.ts

@@ -129,8 +129,10 @@ export async function registerFeatureRoutes(app: FastifyInstance): Promise<void>
 
       const jobId = crypto.randomUUID();
       const scriptPath = join(process.cwd(), "packages/ai/python/install_feature.py");
+      const manifestPath = getManifestPath();
+      const modelsDir = getModelsDir();
 
-      const child = spawn(pythonPath, [scriptPath, bundleId], {
+      const child = spawn(pythonPath, [scriptPath, bundleId, manifestPath, modelsDir], {
         stdio: ["ignore", "ignore", "pipe"],
         env: {
           ...process.env,