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

lib/Ebook.php

@@ -1984,6 +1984,10 @@ final class Ebook{
 
 			$this->RemoveEbookPlaceholder();
 			$this->AddEbookPlaceholder();
+
+			EbookTag::DeleteUnused();
+			LocSubject::DeleteUnused();
+			Collection::DeleteUnused();
 		}
 		catch(Exceptions\ValidationException $ex){
 			$error = new Exceptions\InvalidEbookException();
@@ -2176,6 +2180,10 @@ final class Ebook{
 		$this->RemoveTocEntries();
 		$this->RemoveEbookPlaceholder();
 
+		EbookTag::DeleteUnused();
+		LocSubject::DeleteUnused();
+		Collection::DeleteUnused();
+
 		foreach($this->Projects as $project){
 			$project->Delete();
 		}

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]);
+	}
 }

lib/LocSubject.php

@@ -64,4 +64,16 @@ class LocSubject{
 			return $this;
 		}
 	}
+
+	/**
+	 * Deletes `LocSubject`s that are not associated with any `Ebook`s.
+	 */
+	public static function DeleteUnused(): void{
+		Db::Query('
+			DELETE
+			from LocSubjects
+			where LocSubjectId not in
+				(select distinct LocSubjectId from EbookLocSubjects)
+		');
+	}
 }