feat(O7): availability metrics and dependency health checks

- Health endpoint now checks DB connectivity
- Track health check total/failures
- DEP_DB_UP gauge (1=up, 0=down) + latency histogram
- APP_UPTIME_SECONDS updated on each health check
- Metrics: A1-A10 (10 metrics instrumented)

crates/mem-cli/src/http_server.rs

@@ -453,7 +453,24 @@ pub async fn start_server(port: u16, api_key: String, database_url: &str) -> Res
 
 /// Health check (no auth)
 pub async fn health_check(state: web::Data<AppState>) -> HttpResponse {
+    use crate::metrics::*;
+    HEALTH_CHECKS_TOTAL.inc();
     let uptime = state.start_time.elapsed().as_secs();
+    APP_UPTIME_SECONDS.set(uptime);
+
+    // O7: Check DB dependency
+    let db_start = std::time::Instant::now();
+    match sqlx::query("SELECT 1").execute(&state.pool).await {
+        Ok(_) => {
+            DEP_DB_UP.set(1);
+            DEP_DB_LATENCY.observe(db_start.elapsed().as_secs_f64());
+        }
+        Err(_) => {
+            DEP_DB_UP.set(0);
+            HEALTH_CHECK_FAILURES.inc();
+        }
+    }
+
     HttpResponse::Ok().json(json!({"status": "ok", "uptime_seconds": uptime}))
 }