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/pipeline.py

@@ -1,7 +1,9 @@
 """Compose Pose, tracking, evidence policy, and temporal fall state."""
 
+import itertools
 from dataclasses import dataclass
-from typing import Dict, Sequence, Tuple
+from datetime import datetime
+from typing import Dict, Optional, Sequence, Tuple
 
 from v1.config import AppConfig
 from v1.evidence import PoseEvidence, assess_pose_quality, extract_evidence
@@ -11,6 +13,17 @@ from v1.tracking import PersonTracker, TrackedPersonPose
 from v1.video_source import FramePacket, SourceStatus
 
 
+_SESSION_COUNTER = itertools.count(1)
+
+
+def new_session_id() -> str:
+    """Return a process-unique, human-readable run id for event traceability."""
+
+    return "{0}-{1:03d}".format(
+        datetime.now().strftime("%Y%m%d-%H%M%S"), next(_SESSION_COUNTER)
+    )
+
+
 @dataclass(frozen=True)
 class PersonAnalysis:
     tracked_pose: TrackedPersonPose
@@ -47,8 +60,15 @@ class FallPipeline:
         self._active_track_ids = set()
 
     @classmethod
-    def from_config(cls, config: AppConfig, pose_adapter) -> "FallPipeline":
-        """Create one immutable runtime decision flow from validated config."""
+    def from_config(
+        cls, config: AppConfig, pose_adapter, session_id: Optional[str] = None
+    ) -> "FallPipeline":
+        """Create one immutable runtime decision flow from validated config.
+
+        Each run gets a unique ``session_id`` so confirmed-event IDs never collide
+        across monitoring restarts within the same day (no screenshot overwrite or
+        duplicate JSONL identity).
+        """
 
         return cls(
             pose_adapter=pose_adapter,
@@ -59,6 +79,7 @@ class FallPipeline:
                 recovery_window_seconds=config.event.recovery_window_seconds,
                 cooldown_seconds=config.event.cooldown_seconds,
                 config_version=config.runtime_config_version,
+                session_id=session_id or new_session_id(),
             ),
             keypoint_confidence_threshold=config.event.keypoint_confidence_threshold,
         )