Miscellaneous fixes (#22913)

* add log when probing detect stream on startup

when users don't explicitly set detect.width and detect.height, we probe for them. sometimes the probe hangs (camera doesn't support UDP, like some Reolinks), so this log message will make that clearer

* add faq about probing detect stream

* fix stuck activity ring when tracked object transitions to stationary

* drop cache segments past retain cutoff regardless of retention mode

* add maintainer test

frigate/test/test_maintainer.py

@@ -1,3 +1,4 @@
+import datetime
 import sys
 import unittest
 from unittest.mock import MagicMock, patch
@@ -74,6 +75,46 @@ class TestMaintainer(unittest.IsolatedAsyncioTestCase):
                             f"Expected a single warning for unexpected files, got {len(matching)}",
                         )
 
+    async def test_drops_quiet_segment_when_only_motion_retention(self):
+        # Regression: when motion retention is enabled but a segment has no
+        # motion and no review overlaps it, the segment must still be dropped.
+        # Otherwise it sits in cache forever, accumulates, and triggers the
+        # "Unable to keep up with recording segments in cache" warning every
+        # ~10s as the overflow trim in move_files discards the oldest one.
+        config = MagicMock(spec=FrigateConfig)
+
+        camera_config = MagicMock()
+        camera_config.record.enabled = True
+        camera_config.record.continuous.days = 0
+        camera_config.record.motion.days = 1
+        camera_config.record.event_pre_capture = 5
+        config.cameras = {"test_cam": camera_config}
+
+        stop_event = MagicMock()
+        maintainer = RecordingMaintainer(config, stop_event)
+
+        now = datetime.datetime.now(datetime.timezone.utc)
+        start_time = now - datetime.timedelta(seconds=20)
+        end_time = now - datetime.timedelta(seconds=10)
+        cache_path = "/tmp/cache/test_cam@20260417150000+0000.mp4"
+
+        maintainer.end_time_cache = {cache_path: (end_time, 10.0)}
+        # Single processed frame well past end_time with no motion/objects.
+        maintainer.object_recordings_info["test_cam"] = [(now.timestamp(), [], [], [])]
+        maintainer.audio_recordings_info["test_cam"] = []
+
+        maintainer.drop_segment = MagicMock()
+        maintainer.recordings_publisher = MagicMock()
+
+        result = await maintainer.validate_and_move_segment(
+            "test_cam",
+            reviews=[],
+            recording={"start_time": start_time, "cache_path": cache_path},
+        )
+
+        self.assertIsNone(result)
+        maintainer.drop_segment.assert_called_once_with(cache_path)
+
 
 if __name__ == "__main__":
     unittest.main()