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/hybrid_retrieval.rs

@@ -43,7 +43,7 @@ pub struct RankedCandidate {
 pub struct HybridRetriever {
     tfidf_scorer: Arc<mem_core::GlobalTfIdfScorer>,
     semantic_scorer: Arc<mem_core::SemanticScorer>,
-    pipeline: ScoringPipeline,
+    _pipeline: ScoringPipeline,
     min_tfidf_threshold: f32,
     prefilter_limit: usize,
     rrf_tfidf_weight: f32,
@@ -62,7 +62,7 @@ impl HybridRetriever {
         Self {
             tfidf_scorer,
             semantic_scorer,
-            pipeline,
+            _pipeline: pipeline,
             min_tfidf_threshold: 0.3,
             prefilter_limit: 50,
             rrf_tfidf_weight: 0.4,
@@ -71,7 +71,7 @@ impl HybridRetriever {
     }
 
     /// Decide retrieval route based on query and context
-    pub fn route_query(&self, query: &str, has_wiki_scope: bool, is_reference_query: bool) -> RetrievalRoute {
+    pub fn route_query(&self, _query: &str, has_wiki_scope: bool, is_reference_query: bool) -> RetrievalRoute {
         if is_reference_query {
             RetrievalRoute::ReferenceOnly
         } else if has_wiki_scope {