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/http_server.rs

@@ -382,6 +382,7 @@ pub async fn start_server(port: u16, api_key: String, database_url: &str) -> Res
             .app_data(state.clone())
             .wrap(Logger::default())
             .route("/health", web::get().to(health_check))
+            .route("/metrics", web::get().to(crate::metrics::metrics_handler))
             .route("/memory/ingest", web::post().to(ingest_handler))
             .route("/memory/ingest/{ingest_id}", web::get().to(ingest_status))
             .route("/memory/query", web::get().to(query_handler))