Refactor: Modularize middleware and router registration in app/main.py

app/middleware.py

@@ -0,0 +1,12 @@
+import time
+from fastapi import Request
+
+async def add_process_time_header(request: Request, call_next):
+    """
+    Middleware to add the processing time to the response header.
+    """
+    start_time = time.time()
+    response = await call_next(request)
+    process_time = time.time() - start_time
+    response.headers['X-Process-Time'] = str(process_time)
+    return response