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

.env.example

@@ -0,0 +1,24 @@
+# Server
+POSEFIT_WS_HOST=0.0.0.0
+POSEFIT_WS_PORT=8765
+
+# Video processing
+POSEFIT_PROCESS_EVERY_N_FRAMES=1
+
+# Model
+POSEFIT_MODEL_PATH=pose_models/pose_landmarker_full.task
+POSEFIT_PREFER_GPU=1
+
+# Dead bug exercise
+POSEFIT_VISIBILITY_THRESHOLD=0.45
+POSEFIT_EXTENSION_CONFIRM_FRAMES=4
+POSEFIT_RESET_CONFIRM_FRAMES=3
+
+# Audio
+POSEFIT_REP_ANNOUNCER_ENABLED=1
+POSEFIT_REP_ANNOUNCER_RATE=185
+POSEFIT_REP_ANNOUNCER_VOLUME=1.0
+
+# Logging
+POSEFIT_LOG_ROTATION=20 MB
+POSEFIT_LOG_RETENTION=14 days