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/signaling/websocket_server.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+import asyncio
+import json
+
+import websockets
+from loguru import logger
+
+from app.webrtc.peer_session import PeerSession
+from configs.default import WS_HOST, WS_MAX_SIZE, WS_PORT
+
+
+async def handle_client(websocket):
+    client = websocket.remote_address
+    logger.info(f"Client connected: {client}")
+
+    session = PeerSession()
+    await session.handle(websocket)
+
+    logger.info(f"Connection closed: {client}")
+
+
+async def main():
+    logger.info(f"WebRTC signaling server: ws://{WS_HOST}:{WS_PORT}")
+    async with websockets.serve(handle_client, WS_HOST, WS_PORT, max_size=WS_MAX_SIZE):
+        await asyncio.Future()