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/main.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+import os
+
+os.environ["MEDIAPIPE_DISABLE_LOGGING"] = "1"
+os.environ["GLOG_minloglevel"] = "3"
+
+import asyncio
+
+from loguru import logger
+
+from app.core.lifecycle import startup
+from app.signaling.websocket_server import main as serve
+
+
+if __name__ == "__main__":
+    startup()
+    logger.info("Starting server...")
+    asyncio.run(serve())