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/exercises/dead_bug/rules.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from app.exercises.dead_bug.types import DeadBugMetrics, Point
+
+
+def has_required_visibility(landmarks: list[Point], required_indices: tuple[int, ...], visibility_threshold: float) -> bool:
+    return all(landmarks[i].visibility >= visibility_threshold for i in required_indices)
+
+
+def detect_diagonal_extension(metrics: DeadBugMetrics) -> str | None:
+    if metrics.left_leg_extended and metrics.right_leg_extended:
+        return None
+
+    if metrics.right_leg_extended and metrics.left_arm_extended:
+        return "left_arm_right_leg"
+    if metrics.left_leg_extended and metrics.right_arm_extended:
+        return "right_arm_left_leg"
+    return None
+
+
+def is_ready_position(metrics: DeadBugMetrics) -> bool:
+    knees_bent = metrics.left_knee_angle <= 140 and metrics.right_knee_angle <= 140
+    legs_not_extended = not metrics.left_leg_extended and not metrics.right_leg_extended
+    return knees_bent and legs_not_extended and detect_diagonal_extension(metrics) is None