fix: use mtime (not birthtime) for UUID birthday — rsync preserves mtime across systems

src/api/server.rs

@@ -922,7 +922,7 @@ async fn register_single_file(
 
     let birthday = std::fs::metadata(&path)
         .ok()
-        .and_then(|m| m.created().ok())
+        .and_then(|m| m.modified().ok())
         .map(|t| {
             let secs = t.duration_since(std::time::UNIX_EPOCH).unwrap_or_default().as_secs();
             chrono::DateTime::from_timestamp(secs as i64, 0)