Initialize data freshness from Git history

sync.py

@@ -186,6 +186,31 @@ def read_sync_log() -> dict[str, object]:
     return payload if isinstance(payload, dict) else {}
 
 
+def git_data_update_times() -> tuple[str, str] | None:
+    """Return the latest committed data time in UTC and Shanghai time."""
+    try:
+        result = subprocess.run(
+            ["git", "log", "-1", "--format=%cI", "--", str(PUBLISH_PATH)],
+            cwd=ROOT,
+            check=True,
+            text=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+        )
+        committed_at = result.stdout.strip()
+        if not committed_at:
+            return None
+        timestamp = datetime.fromisoformat(committed_at)
+        if timestamp.tzinfo is None:
+            return None
+        return (
+            timestamp.astimezone(timezone.utc).isoformat().replace("+00:00", "Z"),
+            timestamp.astimezone(ZoneInfo("Asia/Shanghai")).isoformat(),
+        )
+    except (OSError, subprocess.SubprocessError, ValueError):
+        return None
+
+
 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 = {
@@ -270,8 +295,13 @@ def sync() -> None:
         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
+            historical_times = git_data_update_times()
-        if not isinstance(last_data_update_at_shanghai, str) or not last_data_update_at_shanghai:
+            if historical_times is None:
+                last_data_update_at = None
+                last_data_update_at_shanghai = None
+            else:
+                last_data_update_at, last_data_update_at_shanghai = historical_times
+        elif 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"),