Fix deadlock in spatial_scene — lock re-entrancy

observe() held self._lock, called _check_anomaly, which called
get_usual_direction, which tried to acquire self._lock again → deadlock.
Split into _usual_direction_unlocked (no lock) for internal use.

This caused /scene and all other API endpoints to hang after the first
sound classification with spatial data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spatial_scene.py

@@ -168,8 +168,8 @@ class SpatialScene:
         if now - self._last_anomaly.get(category, 0) < 30.0:
             return None
 
-        # Find the usual direction for this category
-        usual_angle = self.get_usual_direction(category)
+        # Find the usual direction for this category (already holding lock)
+        usual_angle = self._usual_direction_unlocked(category)
         if usual_angle is None:
             return None
 
@@ -192,14 +192,12 @@ class SpatialScene:
 
         return None
 
-    def get_usual_direction(self, category: str) -> Optional[float]:
-        """Get the most common direction for a sound category (weighted average)."""
-        with self._lock:
+    def _usual_direction_unlocked(self, category: str) -> Optional[float]:
+        """Get the most common direction for a category. Caller must hold self._lock."""
         bins = self.scene_map.get(category)
         if not bins:
             return None
 
-            # Weighted circular mean
         total_weight = sum(bins.values())
         if total_weight == 0:
             return None
@@ -211,15 +209,19 @@ class SpatialScene:
             sin_sum += count * math.sin(angle_rad)
             cos_sum += count * math.cos(angle_rad)
 
-            mean_angle = math.degrees(math.atan2(sin_sum, cos_sum)) % 360
-            return mean_angle
+        return math.degrees(math.atan2(sin_sum, cos_sum)) % 360
+
+    def get_usual_direction(self, category: str) -> Optional[float]:
+        """Get the most common direction for a sound category (thread-safe)."""
+        with self._lock:
+            return self._usual_direction_unlocked(category)
 
     def get_scene_summary(self) -> dict:
         """Get a summary of the learned spatial scene."""
         with self._lock:
             summary = {}
             for category in sorted(self.scene_map.keys()):
-                usual = self.get_usual_direction(category)
+                usual = self._usual_direction_unlocked(category)
                 total = self.category_totals[category]
                 if usual is not None:
                     summary[category] = {