Add PromQL query support for dashboard visualizations (ticket 013)

Hand-rolls a minimal PromQL parser (src/promql.rs) for metric-name +
label-matcher selection, optional [range] selectors, and top-level
sum()/avg()/rate() wrapping, rather than pulling in a full PromQL parser
crate — the required subset is small and a hand-rolled parser stays fully
engine-independent (mirrors filter_parser.rs) with no unused-AST overhead.

Adds EngineQuery::Promql/EngineResponse::Promql (engine parses + evaluates
in place against the index, no copying) and GET /api/query, returning
Vec<PromqlSeriesDto> (labels + (timestamp_ms, f64) points) for charting,
with malformed queries surfaced as 400 + ErrorDto.

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

src/dto.rs

@@ -83,6 +83,31 @@ pub struct BucketValuesParams {
     pub limit: Option<usize>,
 }
 
+/// One chart-ready time series returned by `GET /api/query` (ticket 013's PromQL
+/// endpoint): a label combination plus its `(timestamp_ms, value)` points, sorted by
+/// timestamp. `labels` uses plain `String` values (not `MetricPrimitive`) since every
+/// value has already been through `MetricPrimitive`'s `Display` form — chart code
+/// doesn't need the typed value, just something to show in a legend/tooltip.
+///
+/// `value` is always `f64` (see `crate::engine::coerce_to_f64` for how a bucket's
+/// `MetricPrimitive` is coerced into one), since a chart needs a plottable number
+/// regardless of the underlying metric primitive's kind.
+#[derive(Debug, Clone, Serialize, PartialEq)]
+pub struct PromqlSeriesDto {
+    pub labels: BTreeMap<String, String>,
+    pub points: Vec<(i64, f64)>,
+}
+
+/// Query params for `GET /api/query`. `promql` is the query string (see
+/// `crate::promql`); `start_ms`/`end_ms` bound which points are included, mirroring
+/// `/api/logs`/`/api/metrics/:name`.
+#[derive(Debug, Deserialize)]
+pub struct PromqlParams {
+    pub promql: String,
+    pub start_ms: Option<i64>,
+    pub end_ms: Option<i64>,
+}
+
 /// JSON error body returned by API handlers on a 4xx/5xx response, e.g.
 /// `{ "error": "invalid regex: ..." }`.
 #[derive(Debug, Serialize)]