feat: add proquint-based userid generation

tests/test_userid.py

@@ -0,0 +1,20 @@
+from fastapi_oidc_op.userid import generate_userid
+
+
+def test_generate_userid_format() -> None:
+    userid = generate_userid()
+    # 32-bit proquint format: xxxxx-xxxxx
+    parts = userid.split("-")
+    assert len(parts) == 2
+    for part in parts:
+        assert len(part) == 5
+
+
+def test_generate_userid_uniqueness() -> None:
+    ids = {generate_userid() for _ in range(100)}
+    assert len(ids) == 100  # All unique
+
+
+def test_generate_userid_is_lowercase() -> None:
+    userid = generate_userid()
+    assert userid == userid.lower()