Conditional upsert: upload skips edited records, updates not-edited

- Add last_edited_at + last_edited_by columns to holidays + leave_records
- Migration: ALTER TABLE adds columns (idempotent)
- PUT endpoints set last_edited_at on every manual edit
- POST /api/leave/upload: skip if last_edited_at NOT NULL, update otherwise
- New POST /api/holidays/upload: same logic, admin only
- LeaveOut + HolidayOut schemas include last_edited_at
- Response includes added/updated/skipped counters
- Verified: edited Jay Lam SL preserved, not-edited May Chan SL updated

backend/models.py

@@ -187,6 +187,10 @@ class Holiday(Base):
     source = Column(String(50), nullable=True)             # "gov_hk_1823" / "manual"
     notes = Column(Text, nullable=True)
 
+    # 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)
 
@@ -210,6 +214,10 @@ class LeaveRecord(Base):
     source = Column(String(50), nullable=True)              # "csv_upload" / "manual"
     notes = Column(Text, nullable=True)
 
+    # 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_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)