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/config.example.json

@@ -1,7 +1,8 @@
 {
   "source": {
     "id": "lobby-camera-01",
-    "rtsp_url_env": "SILVER_POSE_RTSP_URL"
+    "rtsp_url_env": "SILVER_POSE_RTSP_URL",
+    "mode": "stream"
   },
   "model": {
     "path": "models/best.pt",