Apply conditional upsert to attendance + accident Excel upload

Schema:
- attendance_records: last_edited_at + last_edited_by (was: is_manually_edited boolean only)
- accidents: last_edited_at + last_edited_by (was: no edit tracking)

Migration: ALTER TABLE adds columns (idempotent)

Logic:
- Attendance import: skip if last_edited_at IS NOT NULL OR is_manually_edited (backward compat)
- Accident import: skip if last_edited_at IS NOT NULL (was: always overwrite)

PUT endpoints:
- PUT /api/attendance/{record_id}: set last_edited_at + last_edited_by + is_manually_edited
- PUT /api/accident/{record_id} (NEW): set last_edited_at + last_edited_by, whitelist editable fields

Verified via direct _import_attendance_to_db / _import_accident_to_db call:
- Edited attendance: skipped on re-import (status_text preserved)
- Edited accident: skipped on re-import (description preserved)
- Not-edited accident: updated to new values (description changed)

Note: HTTP /api/upload/{section} returned 404 in test, but function logic verified working

backend/models.py

@@ -50,6 +50,11 @@ class Accident(Base):
 
     custom_fields = Column(JSON, default={})
     created_by = Column(Integer, ForeignKey("users.id"))
+
+    # Manual edit tracking - upload skips records with non-null last_edited_at
+    last_edited_at = Column(DateTime, nullable=True)
+    last_edited_by = Column(Integer, ForeignKey("users.id"), nullable=True)
+
     created_at = Column(DateTime, default=now_hkt)
     updated_at = Column(DateTime, default=now_hkt, onupdate=now_hkt)
     deleted_at = Column(DateTime, nullable=True)
@@ -156,12 +161,16 @@ class AttendanceRecord(Base):
     actual_in = Column(String(10), nullable=True)
     actual_out = Column(String(10), nullable=True)
 
-    # 人手修改標記
+    # 人手修改標記 (deprecated - use last_edited_at instead)
     is_manually_edited = Column(Boolean, default=False)
 
     # 原始 Excel 數據
     raw_data = Column(JSON, default={})
 
+    # Manual edit tracking - upload skips records with non-null last_edited_at
+    last_edited_at = Column(DateTime, nullable=True)
+    last_edited_by = Column(Integer, ForeignKey("users.id"), nullable=True)
+
     created_at = Column(DateTime, default=now_hkt)
     updated_at = Column(DateTime, default=now_hkt, onupdate=now_hkt)