Make elapsed-since-last an explicit param extractor, not a Duration-kind special case (ticket 038)

Previously a Value-role param with kind: Duration silently meant "elapsed
time since the previous same-labeled event" instead of parsed text — the
same MetricPrimitiveKind::Duration variant meant two unrelated things
depending on which list (label_params vs value_param) referenced it.

Add ParamDeclaration::elapsed_since_last, a third extraction source
alongside capture_group/json_field_path: when set, the param is computed
(via a new resolve_param_value, used uniformly for label_params and
value_param) rather than text-extracted, and declare_param validates it's
paired with kind: Duration. apply_metric_to_log no longer branches on
(role, kind) — it dispatches purely on what each resolved param declares.

Migrate existing profiles: migrate_duration_value_params converts any
Duration-kind param referenced as a metric's value_param into the new
extractor, and resolve_or_create_legacy_param (ticket 037's pre-existing
migration) now carries the same meaning forward for pre-037 inline
Value-role Duration matchers.

Params.tsx offers "Elapsed since previous event" as its own Extraction
option (hides the now-irrelevant message-match-regex/capture-group
fields and forces kind to duration); Metrics.tsx labels such params
accordingly wherever they're listed as label/value param choices.

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

src/api.rs

@@ -244,6 +244,7 @@ async fn declare_param(
                 capture_group: request.capture_group,
                 json_field_path: request.json_field_path,
                 kind: request.kind,
+                elapsed_since_last: request.elapsed_since_last,
             };
             (StatusCode::CREATED, Json(dto)).into_response()
         }