perf(track): avoid per-frame allocations and list lookups in tracker (#23523)

Two small per-object-per-frame improvements in the tracker hot path
(match_and_update), both bit-identical:

- get_stationary_threshold returned a freshly constructed StationaryThresholds
  (a dataclass plus a list) on every call for any label not in the three
  known lists - i.e. for common labels like person/dog. The default thresholds
  are constant and never mutated, so return a shared module-level singleton,
  as the other three cases already do.
- untracked_object_boxes membership used `box not in [list of boxes]` (O(n));
  build a set of box tuples for O(1) membership. Boxes are hashable as tuples
  and output is unchanged.

get_stationary_threshold appeared in a live py-spy --gil profile of a camera
process_frames worker. Adds tests for the threshold lookups.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frigate/track/stationary_classifier.py

@@ -63,6 +63,9 @@ NON_STATIONARY_OBJECT_THRESHOLDS = StationaryThresholds(
     max_stationary_history=4,
 )
 
+# Default thresholds for any other object label
+DEFAULT_OBJECT_THRESHOLDS = StationaryThresholds()
+
 
 def get_stationary_threshold(label: str) -> StationaryThresholds:
     """Get the stationary thresholds for a given object label."""
@@ -76,7 +79,7 @@ def get_stationary_threshold(label: str) -> StationaryThresholds:
     if label in NON_STATIONARY_OBJECT_THRESHOLDS.objects:
         return NON_STATIONARY_OBJECT_THRESHOLDS
 
-    return StationaryThresholds()
+    return DEFAULT_OBJECT_THRESHOLDS
 
 
 class StationaryMotionClassifier: