Accident: add incident_datetime computed field (date HH:MM:SS)

Backend:
- to_dict() now includes incident_datetime: "YYYY-MM-DD HH:MM:SS"
- Composed from r.date (YYYY-MM-DD) + r.time (HH:MM:SS, padded if HH:MM)
- _get_excel_column_order_accident() adds incident_datetime column after time
- Export xlsx rec dict now includes incident_datetime

Verified:
- /api/accident/records: incident_datetime = "2026-07-30 09:40:00"
- column_order includes incident_datetime column
- /api/accident/export/excel: xlsx 明細 sheet now has incident_datetime column
- Time edge case: HH:MM auto-padded to HH:MM:SS

Frontend:
- Accident list page auto-displays new column via column_order
- No frontend changes needed (renders record[h] as string)

backend/main.py

@@ -2113,6 +2113,7 @@ def _get_excel_column_order_accident() -> list:
         "description",
         "severity",
         "time",
+        "incident_datetime",  # combined "YYYY-MM-DD HH:MM:SS" (computed)
         "location",
         "patient_or_staff",
         "long_description",
@@ -2340,11 +2341,22 @@ async def accident_records(
     rows = q.offset((page - 1) * per_page).limit(per_page).all()
 
     def to_dict(r):
+        # Compose incident_datetime from date + time (display as "YYYY-MM-DD HH:MM:SS")
+        incident_dt = None
+        if r.date:
+            date_part = r.date.isoformat()
+            time_part = r.time or "00:00:00"
+            # Ensure time has HH:MM:SS format (may be HH:MM from Excel import)
+            if len(time_part) == 5:  # "HH:MM"
+                time_part += ":00"
+            incident_dt = f"{date_part} {time_part}"
+
         return {
             "id": r.id,
             "submitted_at": r.submitted_at.isoformat() if r.submitted_at else None,
             "date": r.date.isoformat() if r.date else None,
             "time": r.time,
+            "incident_datetime": incident_dt,  # YYYY-MM-DD HH:MM:SS combined
             "location": r.location,
             "employee_name": r.submitter_email,
             "employee_id": r.injured_person_id,
@@ -2946,11 +2958,21 @@ async def accident_export_excel(
     ws.freeze_panes(1, 0)
 
     for r_idx, r in enumerate(rows, start=1):
+        # Compose incident_datetime from date + time
+        incident_dt = ""
+        if r.date:
+            date_part = r.date.isoformat()
+            time_part = r.time or "00:00:00"
+            if len(time_part) == 5:
+                time_part += ":00"
+            incident_dt = f"{date_part} {time_part}"
+
         rec = {
             "id": r.id,
             "submitted_at": r.submitted_at.isoformat() if r.submitted_at else "",
             "date": r.date.isoformat() if r.date else "",
             "time": r.time or "",
+            "incident_datetime": incident_dt,
             "location": r.location or "",
             "submitter_email": r.submitter_email or "",
             "injured_person_id": r.injured_person_id or "",