fix(tower): lock transfers history writes

Add _history_lock to serialize history load→extend→write in
transfers_end(), preventing lost-update races on transfers.json if a
second writer is added later. Fix file handle leak in
_transfers_history_load() with context manager.

tower/lisael_content.py

@@ -412,6 +412,9 @@ def push_file(ip, name, subdir="podcasts"):
 _busy = set()
 _busy_lock = threading.Lock()
 
+# ---- transfers journal lock ----
+_history_lock = threading.Lock()
+
 def push_batch(ip, delta):
     ok = 0
     for f in delta:
@@ -494,7 +497,8 @@ def transfers_active():
 def _transfers_history_load():
     if os.path.exists(TRANSFERS_JSON):
         try:
-            return json.load(open(TRANSFERS_JSON, encoding="utf-8"))
+            with open(TRANSFERS_JSON, encoding="utf-8") as fh:
+                return json.load(fh)
         except Exception:
             pass
     return []
@@ -510,7 +514,8 @@ def transfers_end():
         done = [{k: v for k, v in f.items() if k != "_t0"}
                 for f in _active["files"] if f["status"] in ("done", "failed")]
         _active = None
-    hist = _transfers_history_load()
-    hist.extend(done)
-    hist = hist[-TRANSFERS_MAX:]
-    _atomic_write_json(TRANSFERS_JSON, hist)
+    with _history_lock:
+        hist = _transfers_history_load()
+        hist.extend(done)
+        hist = hist[-TRANSFERS_MAX:]
+        _atomic_write_json(TRANSFERS_JSON, hist)