fix(v1): wire model confidence, explicit source mode, unique event ids

A: PoseAdapter.set_confidence_threshold is applied on start, so the
   settings model-confidence field actually affects inference.
B: config source.mode ('stream'|'replay') is explicit; app no longer
   guesses the source type from the URL prefix.
C: FallStateMachine takes a session_id and from_config generates a
   unique one per run, so event ids never collide across restarts
   (no screenshot overwrite or duplicate JSONL identity in a day).

51 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v1/pose.py

@@ -66,6 +66,18 @@ class PoseAdapter:
     def model_path(self) -> Path:
         return self._model_path
 
+    @property
+    def confidence_threshold(self) -> float:
+        return self._confidence
+
+    def set_confidence_threshold(self, value: float) -> None:
+        """Update the inference confidence so settings changes take effect."""
+
+        confidence = float(value)
+        if not 0.0 <= confidence <= 1.0:
+            raise ModelValidationError("confidence threshold must be between 0 and 1")
+        self._confidence = confidence
+
     def infer(self, image: np.ndarray) -> Sequence[PersonPose]:
         results = self._model(image, conf=self._confidence, verbose=False)
         return self.from_results(results, self._person_class_ids)