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/webrtc/frame_source.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+import numpy as np
+from loguru import logger
+
+
+TARGET_WIDTH = 1280
+TARGET_HEIGHT = 720
+
+
+def validate_frame_size(image: np.ndarray, width: int = TARGET_WIDTH, height: int = TARGET_HEIGHT) -> None:
+    h, w = image.shape[:2]
+    if w != width or h != height:
+        logger.warning("Unexpected frame size: {}x{}", w, h)