settings: add Clear Tag Category Cache button

behavior change: Settings > Cache now has a 'Clear Tag Category
Cache' action that wipes the per-site tag_types table via the
existing db.clear_tag_cache() hook. This also drops the
__batch_api_probe__ sentinel so Gelbooru/Moebooru sites re-probe
the batch tag API on next use and repopulate the cache from a
fresh response.

Use case: category types like Character/Copyright/Meta appear
missing when the local tag cache was populated by an older build
that didn't map all of Gelbooru's type codes. Clearing lets the
current _GELBOORU_TYPE_MAP re-label tags cleanly instead of
inheriting whatever the old rows said.

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## [Unreleased]
 
+### Added
+- Settings → Cache: **Clear Tag Category Cache** button — wipes the per-site `tag_types` rows (including the `__batch_api_probe__` sentinel) so Gelbooru/Moebooru backends re-probe and re-populate tag categories from scratch. Useful when a stale cache from an earlier build leaves some category types mis-labelled or missing
+
 ### Changed
 - Thumbnail drag-start threshold raised from 10px to 30px to match the rubber band's gate — small mouse wobbles on a thumb no longer trigger a file drag
 

booru_viewer/gui/settings.py

@@ -328,6 +328,20 @@ class SettingsDialog(QDialog):
 
         actions_layout.addLayout(btn_row2)
 
+        btn_row3 = QHBoxLayout()
+
+        clear_tags_btn = QPushButton("Clear Tag Category Cache")
+        clear_tags_btn.setToolTip(
+            "Wipe the per-site tag-type cache (Gelbooru/Moebooru sites). "
+            "Use this if category colors stop appearing correctly — the "
+            "app will re-fetch tag types on the next post view."
+        )
+        clear_tags_btn.clicked.connect(self._clear_tag_cache)
+        btn_row3.addWidget(clear_tags_btn)
+        btn_row3.addStretch(1)
+
+        actions_layout.addLayout(btn_row3)
+
         layout.addWidget(actions_group)
         layout.addStretch()
         return w
@@ -699,6 +713,18 @@ class SettingsDialog(QDialog):
         QMessageBox.information(self, "Done", f"Evicted {count} files.")
         self._refresh_stats()
 
+    def _clear_tag_cache(self) -> None:
+        reply = QMessageBox.question(
+            self, "Confirm",
+            "Wipe the tag category cache for every site? This also clears "
+            "the per-site batch-API probe result, so the app will re-probe "
+            "Gelbooru/Moebooru backends on next use.",
+            QMessageBox.StandardButton.Yes | QMessageBox.StandardButton.No,
+        )
+        if reply == QMessageBox.StandardButton.Yes:
+            count = self._db.clear_tag_cache()
+            QMessageBox.information(self, "Done", f"Deleted {count} tag-type rows.")
+
     def _bl_export(self) -> None:
         from .dialogs import save_file
         path = save_file(self, "Export Blacklist", "blacklist.txt", "Text (*.txt)")