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/Collection.php

@@ -99,6 +99,17 @@ class Collection{
 				', [], Collection::class);
 	}
 
+	/**
+	 * Deletes `Collection`s that no `Ebook`s are members of.
+	 */
+	public static function DeleteUnused(): void{
+		Db::Query('
+			DELETE
+			from Collections
+			where CollectionId not in
+				(select distinct CollectionId from CollectionEbooks)
+		');
+	}
 
 	// *******
 	// METHODS