Refactor into modular app structure

Split monolithic files into focused modules:
- app/core: settings, logging, lifecycle
- app/signaling: websocket server, ICE parser, message models
- app/webrtc: peer session, video receiver, frame source
- app/vision: pose landmarker wrapper, model config, pose types
- app/exercises/dead_bug: detector, metrics, rules, state machine, types
- app/rendering: skeleton renderer, status overlay, window display
- app/audio: rep announcer
- app/diagnostics: perf timer, crash handler
- configs: environment-based settings
- tests: unit tests for rules, state machine, ICE parser
- run.py: entry point

app/vision/pose_models.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+import mediapipe as mp
+
+BaseOptions = mp.tasks.BaseOptions
+
+_MODELS_DIR = Path(__file__).resolve().parent.parent.parent / "pose_models"
+
+POSE_LANDMARKER_FULL = _MODELS_DIR / "pose_landmarker_full.task"
+POSE_LANDMARKER_LITE = _MODELS_DIR / "pose_landmarker_lite.task"
+
+DEFAULT_MODEL_PATH = str(POSE_LANDMARKER_FULL) if POSE_LANDMARKER_FULL.exists() else str(POSE_LANDMARKER_LITE)