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

Cargo.lock

@@ -601,6 +601,7 @@ dependencies = [
  "chrono",
  "clap",
  "directories",
+ "http-body-util",
  "humantime",
  "pest",
  "pest_derive",
@@ -609,6 +610,7 @@ dependencies = [
  "serde_json",
  "tempfile",
  "tokio",
+ "tower",
  "tower-http",
 ]