media_controller: throttle auto_evict_cache to once per 30s

cache_size_bytes() does a full stat() of every file in the cache
directory. It was called on every image load and every infinite
scroll drain. Now skipped if less than 30 seconds since the last
check.

Also replace QPixmap with QImageReader in image_dimensions() to
read width/height from the file header without decoding the full
image into memory.

behavior change: cache eviction checks run at most once per 30s
instead of on every image load. Library image dimensions are read
via QImageReader (header-only) instead of QPixmap (full decode).

booru_viewer/gui/media_controller.py

@@ -72,6 +72,7 @@ class MediaController:
         self._app = app
         self._prefetch_pause = asyncio.Event()
         self._prefetch_pause.set()  # not paused
+        self._last_evict_check = 0.0  # monotonic timestamp
 
     # -- Post activation (media load) --
 
@@ -233,6 +234,11 @@ class MediaController:
     # -- Cache eviction --
 
     def auto_evict_cache(self) -> None:
+        import time
+        now = time.monotonic()
+        if now - self._last_evict_check < 30:
+            return
+        self._last_evict_check = now
         if not self._app._db.get_setting_bool("auto_evict"):
             return
         max_mb = self._app._db.get_setting_int("max_cache_mb")
@@ -258,15 +264,16 @@ class MediaController:
 
     @staticmethod
     def image_dimensions(path: str) -> tuple[int, int]:
-        """Read image width/height from a local file."""
+        """Read image width/height from a local file without decoding pixels."""
         from .media.constants import _is_video
         if _is_video(path):
             return 0, 0
         try:
-            from PySide6.QtGui import QPixmap
-            pix = QPixmap(path)
-            if not pix.isNull():
-                return pix.width(), pix.height()
+            from PySide6.QtGui import QImageReader
+            reader = QImageReader(path)
+            size = reader.size()
+            if size.isValid():
+                return size.width(), size.height()
         except Exception:
             pass
         return 0, 0