fix: sync thread waits for restore to finish before starting

Prevent race condition where sync rsync writes to /data/ while
snapshot_download is still populating it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

entrypoint.py

@@ -345,9 +345,15 @@ def save_and_upload():
     log(f"══ SYNC: done ({elapsed_all:.1f}s) — {uploaded} ok, {failed} failed ══")
 
 
+# Event to signal restore completion (sync must wait)
+restore_done = threading.Event()
+
+
 # ── Sync Thread ───────────────────────────────────────────────────────
 def sync_loop():
-    log("sync thread: waiting 30s before first sync")
+    log("sync thread: waiting for restore to finish ...")
+    restore_done.wait()
+    log("sync thread: restore done, waiting 30s before first sync")
     time.sleep(30)
     cycle = 0
     while True:
@@ -533,6 +539,8 @@ def main():
             log("── background restore complete ──")
         except Exception as e:
             log(f"── background restore error: {e} ──")
+        finally:
+            restore_done.set()  # unblock sync thread
 
     threading.Thread(target=background_restore, daemon=True).start()