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

webapp/src/lib/api.ts

@@ -25,6 +25,8 @@ export interface ParamDeclaration {
   name: string
   message_match_regex: string
   capture_group: string
+  /** Alternative to `capture_group`: a dot-separated JSON field path (e.g. "foo.bar"), read from lines that parse as a JSON object. When set (non-empty), takes precedence over `capture_group`. */
+  json_field_path?: string | null
   kind: 'string' | 'number' | 'timestamp' | 'duration'
 }
 
@@ -48,6 +50,8 @@ export type ParamRole = 'label' | 'value'
 export interface MetricParamMatcher {
   param_name: string
   capture_group: string
+  /** See `ParamDeclaration.json_field_path`. */
+  json_field_path?: string | null
   kind: 'string' | 'number' | 'timestamp' | 'duration'
   /** Defaults to 'label' server-side when omitted. */
   role?: ParamRole