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.py

@@ -31,6 +31,7 @@ class AppConfig:
     confidence_threshold: float
     event: EventConfig
     event_dir: Path
+    source_mode: str = "stream"
 
     @property
     def runtime_config_version(self) -> str:
@@ -113,6 +114,10 @@ def load_config(path: Path) -> AppConfig:
     if not source_url:
         raise ConfigError("missing RTSP environment variable: {0}".format(environment_name))
 
+    source_mode = source.get("mode", "stream")
+    if source_mode not in ("replay", "stream"):
+        raise ConfigError("source.mode must be 'replay' or 'stream'")
+
     model = _mapping(root.get("model"), "model")
     model_sha256 = _text(model.get("sha256"), "model.sha256").lower()
     if not _SHA256.match(model_sha256):
@@ -163,4 +168,5 @@ def load_config(path: Path) -> AppConfig:
         ),
         event=event,
         event_dir=_resolve_path(config_path, artifacts.get("event_dir"), "artifacts.event_dir"),
+        source_mode=source_mode,
     )