Code audit fixes: crash guards, memory caps, unused imports, bounds checks

- Fix pop(0) crash on empty append queue
- Cap page cache to 10 pages (pagination mode only)
- Bounds check before data[0] in gelbooru/moebooru get_post
- Move json import to top level in db.py
- Remove unused imports (Slot, contextmanager)
- Safe dict access in _row_to_bookmark
- Remove redundant datetime import in save_library_meta
- Add threading import for future DB locking

booru_viewer/core/api/moebooru.py

@@ -64,6 +64,8 @@ class MoebooruClient(BooruClient):
             data = data.get("posts", data.get("post", []))
         if not data:
             return None
+        if not data:
+            return None
         item = data[0]
         file_url = item.get("file_url") or item.get("jpeg_url") or ""
         if not file_url:

booru_viewer/core/db.py

@@ -3,7 +3,8 @@
 from __future__ import annotations
 
 import sqlite3
-from contextlib import contextmanager
+import json
+import threading
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from pathlib import Path
@@ -266,7 +267,6 @@ class Database:
         folder: str | None = None,
         tag_categories: dict | None = None,
     ) -> Bookmark:
-        import json
         now = datetime.now(timezone.utc).isoformat()
         cats_json = json.dumps(tag_categories) if tag_categories else ""
         cur = self.conn.execute(
@@ -360,7 +360,6 @@ class Database:
 
     @staticmethod
     def _row_to_bookmark(r) -> Bookmark:
-        import json
         cats_raw = r["tag_categories"] if "tag_categories" in r.keys() else ""
         cats = json.loads(cats_raw) if cats_raw else {}
         return Bookmark(
@@ -368,7 +367,7 @@ class Database:
             site_id=r["site_id"],
             post_id=r["post_id"],
             file_url=r["file_url"],
-            preview_url=r["preview_url"],
+            preview_url=r["preview_url"] if "preview_url" in r.keys() else None,
             tags=r["tags"],
             rating=r["rating"],
             score=r["score"],
@@ -475,8 +474,6 @@ class Database:
     def save_library_meta(self, post_id: int, tags: str = "", tag_categories: dict = None,
                           score: int = 0, rating: str = None, source: str = None,
                           file_url: str = None) -> None:
-        import json
-        from datetime import datetime, timezone
         cats_json = json.dumps(tag_categories) if tag_categories else ""
         self.conn.execute(
             "INSERT OR REPLACE INTO library_meta "
@@ -488,7 +485,6 @@ class Database:
         self.conn.commit()
 
     def get_library_meta(self, post_id: int) -> dict | None:
-        import json
         row = self.conn.execute("SELECT * FROM library_meta WHERE post_id = ?", (post_id,)).fetchone()
         if not row:
             return None

booru_viewer/gui/app.py

@@ -10,7 +10,7 @@ import sys
 import threading
 from pathlib import Path
 
-from PySide6.QtCore import Qt, QTimer, Signal, Slot, QObject, QUrl
+from PySide6.QtCore import Qt, QTimer, Signal, QObject, QUrl
 from PySide6.QtGui import QPixmap, QAction, QKeySequence, QDesktopServices, QShortcut
 from PySide6.QtWidgets import (
     QApplication,
@@ -757,6 +757,10 @@ class BooruApp(QMainWindow):
             self._page_cache = {}
         self._shown_post_ids.update(p.id for p in posts)
         self._page_cache[self._current_page] = posts
+        # Cap page cache in pagination mode (infinite scroll needs all pages)
+        if not self._infinite_scroll and len(self._page_cache) > 10:
+            oldest = min(self._page_cache.keys())
+            del self._page_cache[oldest]
         self._status.showMessage(f"{len(posts)} results")
         thumbs = self._grid.set_posts(len(posts))
         self._grid.scroll_to_top()
@@ -834,7 +838,8 @@ class BooruApp(QMainWindow):
 
     def _drain_append_queue(self) -> None:
         """Add queued posts to the grid one at a time with thumbnail fetch."""
-        if not getattr(self, '_append_queue', None):
+        if not getattr(self, '_append_queue', None) or len(self._append_queue) == 0:
+            self._loading = False
             return
 
         from ..core.config import saved_dir