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/dto.rs

@@ -1,6 +1,8 @@
 use crate::data_value::{MetricPrimitiveKind, RegexToMetricPrimitiveMatcher};
+use crate::metric_primitive::MetricPrimitive;
 use crate::settings::CssEntry;
 use serde::{Deserialize, Serialize};
+use std::collections::BTreeMap;
 
 #[derive(Debug, Clone, Serialize)]
 pub struct LogEntryDto {
@@ -54,6 +56,40 @@ pub struct TimelineParams {
     pub bucket_ms: Option<i64>,
 }
 
+/// One entry from a metric's derived bucket timeseries: the label combination
+/// that bucket represents, plus one `(sequence, timestamp, value)` occurrence
+/// within it.
+#[derive(Debug, Clone, Serialize)]
+pub struct BucketValueDto {
+    pub labels: BTreeMap<String, MetricPrimitive>,
+    pub sequence: u64,
+    pub timestamp_ms: i64,
+    pub value: MetricPrimitive,
+}
+
+/// Query params for `GET /api/metrics/:name`. `labels` is a single
+/// comma-separated `key=value` list (e.g. `?labels=path=/a,duration=5`)
+/// rather than repeated `label.foo=bar` params, since axum's `Query`
+/// extractor needs a fixed/known set of field names — an arbitrary label set
+/// doesn't fit that without a custom deserializer. Every listed label must
+/// match (compared against each label value's `Display` form, see
+/// `MetricPrimitive`); unlisted labels on a bucket are ignored.
+#[derive(Debug, Deserialize)]
+pub struct BucketValuesParams {
+    pub labels: Option<String>,
+    pub since_sequence: Option<u64>,
+    pub start_ms: Option<i64>,
+    pub end_ms: Option<i64>,
+    pub limit: Option<usize>,
+}
+
+/// JSON error body returned by API handlers on a 4xx/5xx response, e.g.
+/// `{ "error": "invalid regex: ..." }`.
+#[derive(Debug, Serialize)]
+pub struct ErrorDto {
+    pub error: String,
+}
+
 /// Request body for `POST /api/settings/css`: raw CSS content to import and
 /// store alongside the settings file.
 #[derive(Debug, Deserialize)]