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/chunk_metadata.rs

@@ -1,18 +1,4 @@
-/// Phase 5: Chunk Metadata Index
-///
-/// Extract and index chunk metadata for improved scoring:
-/// 1. Heading extraction (markdown hierarchy)
-/// 2. Key term extraction (TF-IDF top terms)
-/// 3. Category inference (error|solution|tool|concept)
-/// 4. Metadata-based scoring boost
-///
-/// Benefits:
-/// - Better semantic understanding (category context)
-/// - Faster ranking (metadata pre-computed)
-/// - Query intent matching (match query intent to chunk category)
-
-use anyhow::Result;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashMap;
 
 /// Chunk category for scoring context
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]