fix: zero compiler warnings + readiness endpoint

- Fix all 106 warnings: unused imports, dead struct fields (_prefix),
  dead_code annotations on bin-only items in ingest_worker.rs
- Add GET /ready endpoint (readiness probe, checks DB)
- Keep GET /health lightweight (liveness, no DB check)
- Add ServiceMonitor for Prometheus scraping
- Add tracing-subscriber json feature for structured logs
- 760 tests passing, 0 warnings, 0 errors

crates/mem-cli/src/query/faceted_search.rs

@@ -6,7 +6,6 @@
 use chrono::{DateTime, Timelike, Utc};
 use serde::{Deserialize, Serialize};
 use sqlx::{Pool, Postgres};
-use std::collections::HashMap;
 use tracing::{debug, info};
 
 /// A single facet (filterable dimension)
@@ -88,7 +87,7 @@ impl FacetedSearch {
         limit: usize,
     ) -> Result<AvailableFacets, String> {
         let limit = limit.max(5).min(50);
-        let start_time = std::time::Instant::now();
+        let _start_time = std::time::Instant::now();
 
         debug!("Discovering facets for {}, limit={}", search_type, limit);