feat(auth): implement response_type=id authentication flow

Implements both IndieAuth flows per W3C specification:
- Authentication flow (response_type=id): Code redeemed at authorization endpoint, returns only user identity
- Authorization flow (response_type=code): Code redeemed at token endpoint, returns access token

Changes:
- Authorization endpoint GET: Accept response_type=id (default) and code
- Authorization endpoint POST: Handle code verification for authentication flow
- Token endpoint: Validate response_type=code for authorization flow
- Store response_type in authorization code metadata
- Update metadata endpoint: response_types_supported=[code, id], code_challenge_methods_supported=[S256]

The default behavior now correctly defaults to response_type=id when omitted, per IndieAuth spec section 5.2.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/unit/test_metadata.py

@@ -78,11 +78,11 @@ class TestMetadataEndpoint:
         assert data["token_endpoint"] == "https://auth.example.com/token"
 
     def test_metadata_response_types_supported(self, client):
-        """Test response_types_supported contains only 'code'."""
+        """Test response_types_supported contains both 'code' and 'id'."""
         response = client.get("/.well-known/oauth-authorization-server")
         data = response.json()
 
-        assert data["response_types_supported"] == ["code"]
+        assert data["response_types_supported"] == ["code", "id"]
 
     def test_metadata_grant_types_supported(self, client):
         """Test grant_types_supported contains only 'authorization_code'."""
@@ -91,12 +91,12 @@ class TestMetadataEndpoint:
 
         assert data["grant_types_supported"] == ["authorization_code"]
 
-    def test_metadata_code_challenge_methods_empty(self, client):
-        """Test code_challenge_methods_supported is empty array."""
+    def test_metadata_code_challenge_methods_supported(self, client):
+        """Test code_challenge_methods_supported contains S256."""
         response = client.get("/.well-known/oauth-authorization-server")
         data = response.json()
 
-        assert data["code_challenge_methods_supported"] == []
+        assert data["code_challenge_methods_supported"] == ["S256"]
 
     def test_metadata_token_endpoint_auth_methods(self, client):
         """Test token_endpoint_auth_methods_supported contains 'none'."""