feat(O5): write volume + storage metrics with background collector

- Track entity/edge/chunk writes and errors
- Track bytes written per write operation
- Background task: collect DB row counts every 60s
- Background task: collect pool size/idle stats
- Metrics: W1-W12 (12 metrics instrumented)

crates/mem-cli/src/http_server.rs

@@ -374,6 +374,30 @@ pub async fn start_server(port: u16, api_key: String, database_url: &str) -> Res
     });
 
     tracing::info!("Starting HTTP server on port {}", port);
+
+    // O5/O7/O9: Background stats collector (every 60s)
+    {
+        let stats_pool = state.get_ref().pool.clone();
+        tokio::spawn(async move {
+            let mut interval = tokio::time::interval(std::time::Duration::from_secs(60));
+            loop {
+                interval.tick().await;
+                // O5: Table row counts
+                if let Ok(row) = sqlx::query_as::<_, (i64,)>("SELECT COUNT(*) FROM memory_entity")
+                    .fetch_one(&stats_pool).await {
+                    crate::metrics::DB_TABLE_ENTITY_ROWS.set(row.0 as u64);
+                }
+                if let Ok(row) = sqlx::query_as::<_, (i64,)>("SELECT COUNT(*) FROM memory_edge")
+                    .fetch_one(&stats_pool).await {
+                    crate::metrics::DB_TABLE_EDGE_ROWS.set(row.0 as u64);
+                }
+                // O9: Pool stats
+                crate::metrics::DB_POOL_SIZE.set(stats_pool.size() as u64);
+                crate::metrics::DB_POOL_IDLE.set(stats_pool.num_idle() as u64);
+            }
+        });
+    }
+
     tracing::info!("Creating HttpServer instance...");
 
     let server = HttpServer::new(move || {
@@ -792,8 +816,13 @@ async fn store_compacted_memory(
     .await;
 
     match result {
-        Ok(_) => true,
+        Ok(_) => {
+            crate::metrics::WRITE_CHUNKS_TOTAL.inc();
+            crate::metrics::WRITE_BYTES_TOTAL.inc_by(memory.len() as u64);
+            true
+        }
         Err(e) => {
+            crate::metrics::WRITE_ERRORS_TOTAL.inc();
             tracing::error!("Failed to store compacted memory: {}", e);
             false
         }