fix: add periodic expired session cleanup (hourly)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main.rs

@@ -63,6 +63,27 @@ async fn main() -> anyhow::Result<()> {
                 tracing::warn!("Failed to load provider rate limits from DB: {:?}. Using defaults.", e);
             }
 
+            // Periodic session cleanup (every hour)
+            {
+                let cleanup_pool = state.pool.clone();
+                tokio::spawn(async move {
+                    let mut interval = tokio::time::interval(std::time::Duration::from_secs(3600));
+                    loop {
+                        interval.tick().await;
+                        match crate::db::sessions::delete_expired(&cleanup_pool).await {
+                            Ok(count) => {
+                                if count > 0 {
+                                    tracing::info!(deleted = count, "Cleaned up expired sessions");
+                                }
+                            }
+                            Err(e) => {
+                                tracing::warn!(error = %e, "Failed to clean up expired sessions");
+                            }
+                        }
+                    }
+                });
+            }
+
             let shutdown_pool = state.pool.clone();
             let app = router::build_router(state, &config);