fix: use async Modal interfaces to eliminate blocking warnings in webhook handler

Replace blocking modal.Sandbox.create() and sandbox.set_tags() with
their async equivalents (.aio()) in the async github_webhook handler.
Eliminates AsyncUsageWarning from Modal runtime logs.

Author: manascb1344

app.py

@@ -763,14 +763,14 @@ async def github_webhook(request: Request):
             sandbox_kwargs["region"] = MODAL_REGION
 
         spawn_start = time.monotonic()
-        sandbox = modal.Sandbox.create(
+        sandbox = await modal.Sandbox.create.aio(
             "bash",
             "-c",
             cmd,
             **sandbox_kwargs,
         )
 
-        sandbox.set_tags({"job_id": str(job_id)})
+        await sandbox.set_tags.aio({"job_id": str(job_id)})
         spawn_duration = time.monotonic() - spawn_start
 
         logger.info(