Add per-photo-widget lock (freezes current photo until unlocked)

A "Lock this photo" button in the photos widget's dialog toggles
PhotoWidgetConfig.locked, which suppresses both the timer-elapsed
auto-advance and the advance/back button actions until unlocked. The
Layout tab canvas shows a lock badge on any locked photo widget's box.

server/tests/test_widget_border.py

@@ -37,7 +37,7 @@ def test_set_border_persists(client, db_session):
     assert resp.status_code == 200, resp.text
     assert resp.json() == {
         "id": widget_id, "widget_type": "photos", "x": 0, "y": 0, "w": 8, "h": 5, "sort_order": 0,
-        "border_style": "dashed", "border_thickness": 5, "border_color_index": 3,
+        "border_style": "dashed", "border_thickness": 5, "border_color_index": 3, "locked": False,
     }
     widget = db_session.get(Widget, widget_id)
     assert (widget.border_style, widget.border_thickness, widget.border_color_index) == ("dashed", 5, 3)