feat(O10): Prometheus metrics module + /metrics endpoint

- metrics.rs: Counter, Gauge, GaugeF64, Histogram, LabeledCounter types
- Timer RAII helper for automatic latency observation
- All O1-O9 metric definitions pre-declared (119 metrics total)
- render_metrics() outputs Prometheus text exposition format
- GET /metrics endpoint registered in http_server
- HTTP/LLM/DB latency buckets defined
- 6 unit tests passing

crates/mem-cli/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod endpoints;
 pub mod handlers;
 pub mod http_server;
+pub mod metrics;
 pub mod query;
 pub mod auth;
 pub mod ingest_worker;