fix: third-pass review bugs

- Switch DELETE /export to query param (path param strips leading /)
- Add CropKeyframe Pydantic model for typed keyframe validation
- Convert keyframes to tuples before passing to apply_keyframes_to_jobs
- Remove dead QFrame import from main.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -12,7 +12,7 @@ from pathlib import Path
 
 from PyQt6.QtWidgets import (
     QApplication, QMainWindow, QWidget, QVBoxLayout, QHBoxLayout,
-    QLabel, QPushButton, QLineEdit, QFileDialog, QFrame,
+    QLabel, QPushButton, QLineEdit, QFileDialog,
     QListWidget, QListWidgetItem, QAbstractItemView, QSplitter, QToolTip,
     QComboBox, QCheckBox, QSpinBox, QDoubleSpinBox,
     QMessageBox, QInputDialog,