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/message_models.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class SignalingMessage:
+    type: str
+    sdp: str = ""
+    candidate: str = ""
+    sdpMid: str | None = None
+    sdpMLineIndex: int = 0