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/Cargo.toml

@@ -27,7 +27,7 @@ anyhow = { workspace = true }
 thiserror = { workspace = true }
 clap = { workspace = true }
 tracing = { workspace = true }
-tracing-subscriber = { workspace = true }
+tracing-subscriber = { workspace = true, features = ["json"] }
 time = { workspace = true }
 actix-web = { workspace = true }
 actix-rt = { workspace = true }