Accident schema: rename employee_name -> submitter_email (clearer semantic)

Database migration:
- ALTER TABLE accidents RENAME COLUMN employee_name TO submitter_email
- ALTER TABLE accidents RENAME COLUMN responsible_person TO injured_person
- ALTER TABLE accidents RENAME COLUMN employee_id TO injured_person_id

Reason: Accident.employee_name contained submitter EMAIL (not staff name),
causing confusion when joining with attendance_records.employee_name (real name).
New column names are self-documenting.

API backward-compat:
- All API responses still use "employee_name" / "responsible_person" field names
- Internal SQL mapping translates submitter_email/injured_person -> employee_name/responsible_person
- Frontend code unchanged

Verified:
- /api/accident/1 returns employee_name = submitter_email content
- /api/accident/records returns 2 records OK (was 500 before fix)
- /api/employees/list still returns 12 staff (accident excluded)
- Data preserved (1st record id=1 submitter_email = info@scmedical.hk)

backend/models.py

@@ -25,19 +25,29 @@ class User(Base):
 
 
 class Accident(Base):
+    """意外記錄 Accident Reports
+
+    IMPORTANT — Column naming convention (2026-07-19 cleanup):
+    - submitter_email: The Google Form submitter's email (NOT the injured person)
+    - injured_person: Name of the staff who got hurt (was: responsible_person)
+    - injured_person_id: Employee ID (was: employee_id)
+
+    This naming makes it clear these are different concepts, preventing
+    accidental joins with attendance_records.employee_name (which IS a real name).
+    """
     __tablename__ = "accidents"
     id = Column(Integer, primary_key=True, index=True)
     date = Column(Date, nullable=False)
     time = Column(String(10), nullable=True)
     location = Column(String(255), nullable=False)
-    employee_name = Column(String(255), nullable=False)
-    employee_id = Column(String(100))
+    submitter_email = Column(String(255), nullable=False)
+    injured_person_id = Column(String(100))
     department = Column(String(100))
     description = Column(Text, nullable=False)
     severity = Column(String(50), nullable=False)
     medical_report = Column(Text, nullable=True)
     action_taken = Column(Text, nullable=True)
-    responsible_person = Column(String(255), nullable=True)
+    injured_person = Column(String(255), nullable=True)
 
     # Excel-derived (Google Form) extra columns (full map) -- 2026-07-18
     patient_or_staff = Column(Text, nullable=True)         # Excel col 8