Add upload probe endpoint

app.py

@@ -406,6 +406,23 @@ async def create_job(
     return JSONResponse(_snapshot(job))
 
 
+@app.post("/api/probe-upload")
+async def probe_upload(file: Annotated[UploadFile | None, File()] = None) -> JSONResponse:
+    if file is None:
+        raise HTTPException(status_code=400, detail="Choose a video file first.")
+    total = 0
+    while chunk := await file.read(1024 * 1024):
+        total += len(chunk)
+    return JSONResponse(
+        {
+            "ok": True,
+            "filename": file.filename or "upload.mp4",
+            "content_type": file.content_type or "",
+            "bytes": total,
+        }
+    )
+
+
 @app.get("/api/jobs/{job_id}")
 def get_job(job_id: str) -> JSONResponse:
     with JOBS_LOCK: