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/fall_state.py

@@ -48,6 +48,7 @@ class FallStateMachine:
         recovery_window_seconds: float,
         config_version: str,
         cooldown_seconds: float = 0.0,
+        session_id: str = "",
     ) -> None:
         if not 1.0 <= confirm_window_seconds <= 3.0:
             raise ValueError("confirm_window_seconds must be between 1 and 3 seconds")
@@ -61,6 +62,7 @@ class FallStateMachine:
         self._recovery_window_seconds = float(recovery_window_seconds)
         self._cooldown_seconds = float(cooldown_seconds)
         self._config_version = config_version.strip()
+        self._session_id = str(session_id).strip()
         self._records: Dict[str, _Record] = {}
         self._next_event_number = 1
 
@@ -141,8 +143,9 @@ class FallStateMachine:
     def _new_event(
         self, track_id: str, suspected_at: float, confirmed_at: float
     ) -> FallEvent:
+        prefix = "FALL-{0}-".format(self._session_id) if self._session_id else "FALL-"
         event = FallEvent(
-            event_id="FALL-{0:06d}".format(self._next_event_number),
+            event_id="{0}{1:06d}".format(prefix, self._next_event_number),
             track_id=track_id,
             config_version=self._config_version,
             suspected_at_monotonic=suspected_at,