Harden worker cleanup + reduce cover-loading churn

- Library/metadata workers: delete the worker on its own thread via
  worker.finished -> worker.deleteLater (bound) instead of letting Python GC
  delete it on the GUI thread after untracking.
- Cover templates: compute the transparent-window scan outside the lock so a
  slow first scan never holds the lock and stalls other cover threads.
- Store: larger cover/date batches (500/400/300) so the store loads 3449
  archives with far fewer QThread + ThreadPoolExecutor creations.

app/covers.py

@@ -797,28 +797,30 @@ def _transparent_rect(img):
 def _game_template():
     """Load the PSP game cover template; return ``(template, window)``."""
     global _game_template_cache
-    if _game_template_cache is None:
-        with _game_template_lock:
-            if _game_template_cache is None:
-                try:
-                    from PySide6.QtGui import QImage
-                except Exception:  # noqa: BLE001 - headless API may lack PySide6
-                    _game_template_cache = (None, (0, 0, 0, 0))
-                    return _game_template_cache
-                img = QImage()
-                for path in (
-                    config.COVERS_DIR / "UMD_GAME_COVER_TEMPLATE.png",
-                    config.DATA_DIR / "covers" / "UMD_GAME_COVER_TEMPLATE.png",
-                ):
-                    if path.exists():
-                        img.load(str(path))
-                        if not img.isNull():
-                            break
-                img = img.convertToFormat(QImage.Format_ARGB32) if not img.isNull() else None
-                _game_template_cache = (
-                    img,
-                    _transparent_rect(img) if img is not None and not img.isNull() else (0, 0, 0, 0),
-                )
+    if _game_template_cache is not None:
+        return _game_template_cache
+    try:
+        from PySide6.QtGui import QImage
+    except Exception:  # noqa: BLE001 - headless API may lack PySide6
+        _game_template_cache = (None, (0, 0, 0, 0))
+        return _game_template_cache
+    img = QImage()
+    for path in (
+        config.COVERS_DIR / "UMD_GAME_COVER_TEMPLATE.png",
+        config.DATA_DIR / "covers" / "UMD_GAME_COVER_TEMPLATE.png",
+    ):
+        if path.exists():
+            img.load(str(path))
+            if not img.isNull():
+                break
+    img = img.convertToFormat(QImage.Format_ARGB32) if not img.isNull() else None
+    entry = (
+        img,
+        _transparent_rect(img) if img is not None and not img.isNull() else (0, 0, 0, 0),
+    )
+    with _game_template_lock:
+        if _game_template_cache is None:
+            _game_template_cache = entry
     return _game_template_cache