Fix scrape crash + cover compose responsiveness

- Cache the UMD/game cover templates AND their transparent-window rects
  (computed once, under a lock). The old code re-ran an O(w*h) Python pixel
  scan on every composited cover, stalling video cover loading and scraping
  for ~1s per cover with GIL contention.
- ScrapeDialog: ignore stale search/scrape results via a generation counter
  and cancel in-flight workers when a new search/scrape starts; clear threads
  on close.
- MetadataWorker (library details): cancel stale metadata fetches and delete
  finished workers/threads instead of leaking one per item selection.

app/covers.py

@@ -773,11 +773,16 @@ def save_cover(release_id, system, data):
 # --------------------------------------------------------------------------- #
 # Game cover compositing (IGDB key art -> full PSP cover)
 # --------------------------------------------------------------------------- #
-_game_template_image = None
+_game_template_cache = None  # (template QImage or None, window (x, y, w, h))
+_game_template_lock = threading.Lock()
 
 
 def _transparent_rect(img):
-    """Bounding box of the fully-transparent pixels in `img`."""
+    """Bounding box of the fully-transparent pixels in `img`.
+
+    Run once per template (the result is cached) — an O(w*h) scan is far too
+    slow to repeat for every composed cover.
+    """
     minx, miny, maxx, maxy = img.width(), img.height(), -1, -1
     for y in range(img.height()):
         for x in range(img.width()):
@@ -790,24 +795,31 @@ def _transparent_rect(img):
 
 
 def _game_template():
-    """Load the PSP game cover template (top header + transparent window)."""
-    global _game_template_image
-    if _game_template_image is None:
-        try:
-            from PySide6.QtGui import QImage
-        except Exception:  # noqa: BLE001 - headless API may lack PySide6
-            return None
-        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
-        _game_template_image = img.convertToFormat(QImage.Format_ARGB32) if not img.isNull() else None
-    return _game_template_image
+    """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),
+                )
+    return _game_template_cache
 
 
 def compose_game_cover(key_art_bytes):
@@ -822,14 +834,13 @@ def compose_game_cover(key_art_bytes):
     except Exception:  # noqa: BLE001
         return key_art_bytes
 
-    template = _game_template()
+    template, (x, y, w, h) = _game_template()
     art = QImage.fromData(key_art_bytes)
     if template is None or template.isNull() or art.isNull():
         return key_art_bytes
     art = art.convertToFormat(QImage.Format_ARGB32)
     out = QImage(template.size(), QImage.Format_ARGB32)
     out.fill(Qt.transparent)
-    x, y, w, h = _transparent_rect(template)
 
     painter = QPainter(out)
     if w > 0 and h > 0: