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_config.py

@@ -56,6 +56,40 @@ def test_load_config_rejects_embedded_source_address(tmp_path):
         load_config(config_file)
 
 
+def test_source_mode_defaults_to_stream(tmp_path, monkeypatch):
+    config_file = tmp_path / "config.json"
+    _write_config(
+        config_file,
+        {"id": "lobby-camera-01", "rtsp_url_env": "SILVER_POSE_RTSP_URL"},
+    )
+    monkeypatch.setenv("SILVER_POSE_RTSP_URL", "rtsp://demo.invalid/live")
+
+    assert load_config(config_file).source_mode == "stream"
+
+
+def test_source_mode_replay_is_parsed(tmp_path, monkeypatch):
+    config_file = tmp_path / "config.json"
+    _write_config(
+        config_file,
+        {"id": "lobby-camera-01", "rtsp_url_env": "SILVER_POSE_RTSP_URL", "mode": "replay"},
+    )
+    monkeypatch.setenv("SILVER_POSE_RTSP_URL", "rtsp://demo.invalid/live")
+
+    assert load_config(config_file).source_mode == "replay"
+
+
+def test_invalid_source_mode_is_rejected(tmp_path, monkeypatch):
+    config_file = tmp_path / "config.json"
+    _write_config(
+        config_file,
+        {"id": "lobby-camera-01", "rtsp_url_env": "SILVER_POSE_RTSP_URL", "mode": "loop"},
+    )
+    monkeypatch.setenv("SILVER_POSE_RTSP_URL", "rtsp://demo.invalid/live")
+
+    with pytest.raises(ConfigError, match="mode"):
+        load_config(config_file)
+
+
 def test_runtime_config_version_is_stable_and_excludes_rtsp_address(tmp_path, monkeypatch):
     config_file = tmp_path / "config.json"
     _write_config(