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/app.py

@@ -34,7 +34,7 @@ class FrameWorker(QtCore.QThread):
         self._stop = False
 
     def run(self) -> None:
-        mode = SourceMode.STREAM if self._config.source_url.startswith("rtsp") else SourceMode.REPLAY
+        mode = SourceMode.STREAM if self._config.source_mode == "stream" else SourceMode.REPLAY
         source = VideoSource(self._config.source_url, mode=mode)
         pipeline = FallPipeline.from_config(self._config, self._pose_adapter)
         try:
@@ -103,6 +103,7 @@ class ApplicationController:
 
         self._draft.start_monitoring()
         running = _running_config(self._config, self._draft)
+        self._pose_adapter.set_confidence_threshold(running.confidence_threshold)
         writer = EventArtifactWriter(running.event_dir, running.source_id)
         self._dispatcher = AlertDispatcher(writer, QtAlertSink(self.window))
         worker = FrameWorker(running, self._pose_adapter)