feat: add provision_oauth_user() to users module

Creates or updates a user from an OAuth2 provider: new users are
inserted with an empty password_hash (OAuth-only login); existing users
have their display name and avatar refreshed while all other attributes
(admin flag, password_hash, notification_channels) are preserved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_oauth.py

@@ -64,3 +64,55 @@ def test_validate_state_expired(monkeypatch):
     # Wind expiry into the past
     monkeypatch.setitem(oauth._states, state, time_mod.time() - 1000)
     assert oauth.validate_state(state) is False
+
+
+from hbd.server import users as users_mod
+from hbd.server.users import User
+
+
+def _reset_users(entries=None):
+    users_mod.users = entries or {}
+
+
+def test_provision_oauth_user_new():
+    _reset_users()
+    user = users_mod.provision_oauth_user("gituser", "Git User", "https://example.com/avatar.png")
+    assert user.username == "gituser"
+    assert user.full_name == "Git User"
+    assert user.avatar == "https://example.com/avatar.png"
+    assert user.admin is False
+    assert user.password_hash == ""
+    assert "gituser" in users_mod.users
+
+
+def test_provision_oauth_user_no_password_login():
+    _reset_users()
+    user = users_mod.provision_oauth_user("gituser", "Git User", "")
+    assert user.check_password("anything") is False
+
+
+def test_provision_oauth_user_existing_updates_profile():
+    existing = User(
+        username="alice",
+        full_name="Old Name",
+        avatar="old.png",
+        password_hash="pbkdf2:sha256:1:salt:abc",
+        admin=True,
+        notification_channels=["chan1"],
+    )
+    _reset_users({"alice": existing})
+    user = users_mod.provision_oauth_user("alice", "New Name", "new.png")
+    assert user.full_name == "New Name"
+    assert user.avatar == "new.png"
+    # Preserved
+    assert user.admin is True
+    assert user.password_hash == "pbkdf2:sha256:1:salt:abc"
+    assert user.notification_channels == ["chan1"]
+
+
+def test_provision_oauth_user_does_not_overwrite_with_empty():
+    existing = User(username="bob", full_name="Bob", avatar="bob.png")
+    _reset_users({"bob": existing})
+    user = users_mod.provision_oauth_user("bob", "", "")
+    assert user.full_name == "Bob"
+    assert user.avatar == "bob.png"