debug: add request-logging middleware + make upload handler sync

- HTTP middleware logs every incoming request (method, path, body size)
  BEFORE handler runs — catches silent failures at the network/parser layer
- Changed upload handler from async to sync so FastAPI runs it
  in a thread pool, preventing event loop blockage during large
  file reads

frontend/app.py

@@ -40,6 +40,18 @@ from api_client import ApiClient, ApiError, BACKEND_URL
 # ---------------------------------------------------------------------------
 app = FastAPI(title="VM Bench — Frontend", version="1.0.0")
 
+# ── Request logging middleware (logs before handler, catches silent failures) ──
+@app.middleware("http")
+async def log_requests(request: Request, call_next):
+    cl = request.headers.get("content-length", "?")
+    logger.info("→ %s %s (body %s bytes)", request.method, request.url.path, cl)
+    try:
+        response = await call_next(request)
+        return response
+    except Exception:
+        logger.exception("← %s %s FAILED", request.method, request.url.path)
+        raise
+
 BASE = Path(__file__).parent
 app.mount("/static", StaticFiles(directory=str(BASE / "static")), name="static")
 
@@ -117,7 +129,7 @@ async def index(request: Request, vmid: Optional[int] = None, source: Optional[s
 
 
 @app.post("/session/upload")
-async def session_upload(
+def session_upload(
     request: Request,
     vmid: int = Form(...),
     vm_name: str = Form(""),