fix: clean up stale active sessions on startup

After an unclean shutdown, sessions could be left with disconnected_at
NULL, appearing permanently active. Add CloseActiveSessions to the Store
interface and call it at startup to close any leftover sessions.

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

internal/storage/store_test.go

@@ -316,6 +316,51 @@ func TestSessionEvents(t *testing.T) {
 	})
 }
 
+func TestCloseActiveSessions(t *testing.T) {
+	testStores(t, func(t *testing.T, newStore storeFactory) {
+		t.Run("no active sessions", func(t *testing.T) {
+			store := newStore(t)
+			ctx := context.Background()
+
+			n, err := store.CloseActiveSessions(ctx, time.Now())
+			if err != nil {
+				t.Fatalf("CloseActiveSessions: %v", err)
+			}
+			if n != 0 {
+				t.Errorf("closed %d, want 0", n)
+			}
+		})
+
+		t.Run("closes only active sessions", func(t *testing.T) {
+			store := newStore(t)
+			ctx := context.Background()
+
+			// Create 3 sessions: end one, leave two active.
+			id1, _ := store.CreateSession(ctx, "10.0.0.1", "root", "bash")
+			store.CreateSession(ctx, "10.0.0.2", "admin", "bash")
+			store.CreateSession(ctx, "10.0.0.3", "test", "bash")
+			store.EndSession(ctx, id1, time.Now())
+
+			n, err := store.CloseActiveSessions(ctx, time.Now())
+			if err != nil {
+				t.Fatalf("CloseActiveSessions: %v", err)
+			}
+			if n != 2 {
+				t.Errorf("closed %d, want 2", n)
+			}
+
+			// Verify no active sessions remain.
+			active, err := store.GetRecentSessions(ctx, 10, true)
+			if err != nil {
+				t.Fatalf("GetRecentSessions: %v", err)
+			}
+			if len(active) != 0 {
+				t.Errorf("active sessions = %d, want 0", len(active))
+			}
+		})
+	})
+}
+
 func TestGetRecentSessions(t *testing.T) {
 	testStores(t, func(t *testing.T, newStore storeFactory) {
 		t.Run("empty", func(t *testing.T) {