Clean up unused Tags, LocSubjects, and Collections

On `Ebook::Save()` and `Ebook::Delete()`, remove any unreferenced `Tag`,
`LocSubject`, and `Collection` records. These are analogous to these lines in
`Ebook::Create()`:

```
$this->CreateTags();
$this->CreateLocSubjects();
$this->CreateCollections();
```

`EbookPlaceholder`s can't have `Tags` or `LocSubjects` at the moment, but other
mistakes in production that are later corrected could leave unused `Tags` and
`LocSubjects`.

Context: https://github.com/standardebooks/web/pull/447#issuecomment-2555734692

lib/EbookTag.php

@@ -107,4 +107,17 @@ class EbookTag extends Tag{
 
 		return $tags;
 	}
+
+	/**
+	 * Deletes `EbookTag`s that are not associated with any `Ebook`s.
+	 */
+	public static function DeleteUnused(): void{
+		Db::Query('
+			DELETE
+			from Tags
+			where Type = ?
+				and TagId not in
+					(select distinct TagId from EbookTags)
+		', [Enums\TagType::Ebook]);
+	}
 }