Add JSON field path extraction for params/metrics (ticket 015)

RegexToMetricPrimitiveMatcher and the standalone param declaration
(DeclareParamRequest/DataValue::ParamDeclaration) gain an optional
json_field_path, an alternative to capture_group that reads a
dot-separated field path (e.g. "foo.bar") out of lines that parse as
JSON objects. It's additive and serde-defaulted, so existing
regex-based params/metrics are unaffected. Params.tsx now offers a
"JSON field" extraction option when the message match regex looks
like a JSON object.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/api.rs

@@ -223,6 +223,7 @@ async fn declare_param(
                 name: request.name,
                 message_match_regex: request.message_match_regex,
                 capture_group: request.capture_group,
+                json_field_path: request.json_field_path,
                 kind: request.kind,
             };
             (StatusCode::CREATED, Json(dto)).into_response()