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>

hbd/server/config.py

@@ -34,6 +34,9 @@ SERVER_DEFAULTS = {
     "users": {},                # username -> {full_name, avatar, password, admin, notification_channels}
     "default_owner": None,      # Username that owns hosts with no explicit owner
 
+    # OAuth2 providers
+    "oauth": {},                 # oauth.gitea.{url,client_id,client_secret}
+
     # Host management
     "hosts": {},                # Unified host definitions
     "dyndnshosts": [],          # Hosts with dynamic DNS (legacy)