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-store/src/rebuild.rs

@@ -12,7 +12,7 @@ use mem_ingest::OptimizationMetrics;
 
 /// Memory record from log (local copy for rebuild purposes)
 #[derive(Debug, Clone, Serialize, Deserialize)]
-struct MemoryRecord {
+pub struct MemoryRecord {
     pub level: String,
     pub project: String,
     pub query_id: Option<String>,
@@ -25,7 +25,7 @@ struct MemoryRecord {
 
 /// Parent reference for provenance
 #[derive(Debug, Clone, Serialize, Deserialize)]
-struct MemoryParent {
+pub struct MemoryParent {
     pub source: String,
     pub t: i32,
     pub description: Option<String>,