feat: add proquint-based userid generation

src/fastapi_oidc_op/userid.py

@@ -0,0 +1,11 @@
+import secrets
+
+from proquint import uint2quint
+
+
+def generate_userid() -> str:
+    """Generate a unique user identifier in proquint format.
+
+    Returns a 32-bit proquint string like 'lusab-bansen'.
+    """
+    return uint2quint(secrets.randbits(32))