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
Also added `GetFoo()` methods for all the derived properties like `GetUrl()`, `GetHasDownloads()`, etc. Removed that logic from the constructor so that it's reusable in `Ebook::FromFilesystem()` and `Ebook::GetByIdentifier()`
