Show sync freshness status

sync.py

@@ -174,6 +174,18 @@ def run_git(args: list[str], env: dict[str, str]) -> subprocess.CompletedProcess
     )
 
 
+def read_sync_log() -> dict[str, object]:
+    """Read the previously published sync log when it is available."""
+    path = ROOT / SYNC_LOG_PATH
+    if not path.exists():
+        return {}
+    try:
+        payload = json.loads(path.read_text(encoding="utf-8"))
+    except (OSError, json.JSONDecodeError):
+        return {}
+    return payload if isinstance(payload, dict) else {}
+
+
 def publish(content: bytes, sync_log: dict[str, object]) -> None:
     log_content = (json.dumps(sync_log, ensure_ascii=False, indent=2) + "\n").encode("utf-8")
     targets = {
@@ -244,10 +256,28 @@ def sync() -> None:
             sheets.append((name, headers, rows))
     headers, rows = merge_sheets(sheets)
     content = render_html(headers, rows)
+    previous_sync_log = read_sync_log()
+    previous_data_path = ROOT / PUBLISH_PATH
+    data_changed = (
+        not previous_data_path.exists()
+        or previous_data_path.read_bytes() != content
+    )
     success_at = datetime.now(timezone.utc)
+    if data_changed:
+        last_data_update_at = success_at.isoformat().replace("+00:00", "Z")
+        last_data_update_at_shanghai = success_at.astimezone(ZoneInfo("Asia/Shanghai")).isoformat()
+    else:
+        last_data_update_at = previous_sync_log.get("last_data_update_at")
+        last_data_update_at_shanghai = previous_sync_log.get("last_data_update_at_shanghai")
+        if not isinstance(last_data_update_at, str) or not last_data_update_at:
+            last_data_update_at = None
+        if not isinstance(last_data_update_at_shanghai, str) or not last_data_update_at_shanghai:
+            last_data_update_at_shanghai = None
     sync_log = {
         "last_success_at": success_at.isoformat().replace("+00:00", "Z"),
         "last_success_at_shanghai": success_at.astimezone(ZoneInfo("Asia/Shanghai")).isoformat(),
+        "last_data_update_at": last_data_update_at,
+        "last_data_update_at_shanghai": last_data_update_at_shanghai,
         "worksheet_count": len(worksheet_names),
         "record_count": len(rows),
         "artifact_bytes": len(content),