feat: add oauth module skeleton and is_enabled()

Add hbd/server/oauth.py with OAuthError, _gitea_cfg(), and is_enabled()
to detect when all three required Gitea OAuth2 config keys are present.
Add "oauth": {} default to SERVER_DEFAULTS in config.py.

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

tests/test_oauth.py

@@ -0,0 +1,27 @@
+import pytest
+from hbd.server import oauth
+
+
+CFG_OFF = {}
+CFG_ON = {
+    "oauth": {
+        "gitea": {
+            "url": "https://git.example.com",
+            "client_id": "cid",
+            "client_secret": "csec",
+        }
+    }
+}
+CFG_PARTIAL = {"oauth": {"gitea": {"url": "https://git.example.com"}}}
+
+
+def test_is_enabled_when_all_keys_present():
+    assert oauth.is_enabled(CFG_ON) is True
+
+
+def test_is_enabled_false_when_no_oauth_key():
+    assert oauth.is_enabled(CFG_OFF) is False
+
+
+def test_is_enabled_false_when_partial_config():
+    assert oauth.is_enabled(CFG_PARTIAL) is False