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/core/lifecycle.py

@@ -0,0 +1,10 @@
+from __future__ import annotations
+
+from app.diagnostics.crash_handler import enable_crash_handler
+from configs.default import LOG_DIR
+
+
+def startup() -> None:
+    enable_crash_handler(LOG_DIR)
+    from app.core.logging import setup_logging
+    setup_logging()

app/core/logging.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from loguru import logger
+
+from configs.default import LOG_DIR, LOG_RETENTION, LOG_ROTATION
+
+
+def setup_logging() -> None:
+    LOG_DIR.mkdir(parents=True, exist_ok=True)
+
+    logger.add(
+        LOG_DIR / "posefit-server_{time:YYYY-MM-DD}.log",
+        rotation=LOG_ROTATION,
+        retention=LOG_RETENTION,
+        enqueue=True,
+        backtrace=True,
+        diagnose=True,
+    )