Add Metrics API: declare/list params and metrics, query bucket values (ticket 004)

Exposes param/metric declarations and derived bucket data over HTTP:
POST /api/params and POST /api/metrics validate and declare against the
engine, persisting into the active profile (if any) so declarations survive
a restart; GET /api/metrics lists declarations; GET /api/metrics/:name
queries bucket values with label, time-range, sequence, and limit filters.
Adds a new EngineQuery::BucketValues variant that layers this filtering on
top of ticket 003's bucket_values helper.

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

src/data_value.rs

@@ -1,18 +1,23 @@
 use crate::metric_primitive::MetricPrimitive;
 use crate::timestamp::Timestamp;
-use serde::Serialize;
+use serde::{Deserialize, Serialize};
 
 /// A single parsed metric primitive extracted from a regex capture group.
 #[allow(dead_code)]
-#[derive(Debug, Clone, Serialize)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct RegexToMetricPrimitiveMatcher {
     pub param_name: String,
     pub capture_group: String,
     pub kind: MetricPrimitiveKind,
 }
 
+/// `rename_all = "lowercase"` matches every other `kind`-style enum exposed
+/// over the API/settings JSON (`ColorScheme`, `CssEntryKind`,
+/// `ProfileMetricPrimitiveKind`), so `POST /api/params`/`POST /api/metrics`
+/// bodies use `"string"`/`"number"`/`"timestamp"`.
 #[allow(dead_code)]
-#[derive(Debug, Clone, Copy, Serialize)]
+#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
 pub enum MetricPrimitiveKind {
     String,
     Number,