fix(tests): thread cleanup on assertion failure, reduce oversized image

- test_health_monitor.py: wrap start_monitoring calls in try/finally so
  the background thread is always stopped even when an assertion fails
- test_scroll_helper.py: reduce 50,000px test image to 5,000px to avoid
  unnecessary memory pressure on Raspberry Pi

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

test/test_health_monitor.py

@@ -280,8 +280,10 @@ class TestGetRecoverySuggestions:
 class TestMonitorLifecycle:
     def test_start_monitoring(self, monitor):
         monitor.start_monitoring()
+        try:
             assert monitor._monitor_thread is not None
             assert monitor._monitor_thread.is_alive()
+        finally:
             monitor.stop_monitoring()
 
     def test_stop_monitoring(self, monitor):
@@ -292,9 +294,11 @@ class TestMonitorLifecycle:
 
     def test_double_start_no_duplicate_threads(self, monitor):
         monitor.start_monitoring()
+        try:
             thread1 = monitor._monitor_thread
             monitor.start_monitoring()  # should be idempotent
             assert monitor._monitor_thread is thread1
+        finally:
             monitor.stop_monitoring()
 
     def test_register_health_check(self, monitor):

test/test_scroll_helper.py

@@ -224,7 +224,7 @@ class TestCalculateDynamicDuration:
         assert result >= 60
 
     def test_respects_max_duration(self, helper):
-        helper.create_scrolling_image([_make_image(width=50000)])
+        helper.create_scrolling_image([_make_image(width=5000)])
         helper.min_duration = 10
         helper.max_duration = 60
         helper.scroll_speed = 1.0  # very slow → very long time