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/tests/test_pipeline.py

@@ -119,3 +119,18 @@ def test_pipeline_from_config_uses_runtime_version_for_confirmed_event():
     result = pipeline.process(_packet(1.1))
 
     assert result.events[0].config_version == config.runtime_config_version
+
+
+def test_from_config_gives_each_run_a_unique_event_id():
+    config = _config()
+
+    def confirm(pipeline):
+        pipeline.process(_packet(0.0))
+        pipeline.process(_packet(0.1))
+        return pipeline.process(_packet(1.1)).events[0].event_id
+
+    frames = [(_pose(),), (_pose(horizontal=True),), (_pose(horizontal=True),)]
+    first = confirm(FallPipeline.from_config(config, _SequencePoseAdapter(list(frames))))
+    second = confirm(FallPipeline.from_config(config, _SequencePoseAdapter(list(frames))))
+
+    assert first != second