Add Ebook::DownloadUrl for web and feed downloads

This commit adds a rewrite rule for ebook downloads of the form:

```
/ebooks/some-author/some-book/downloads/some-filename.epub
```

to `www/ebooks/download.php`. That file handles the logic of whether to
show a thank you page before beginning the download.

Download URLs in RSS, Atom, and OPDS feeds follow the same pattern, but
they have a query string parameter `?source=feed` to always skip the
thank you page.

lib/Enums/EbookDownloadSource.php

@@ -0,0 +1,7 @@
+<?
+namespace Enums;
+
+enum EbookDownloadSource: string{
+	case Feed = 'feed';
+	case DownloadPage = 'download';
+}

lib/Enums/EbookFormatType.php

@@ -13,4 +13,27 @@ enum EbookFormatType: string{
 			default => 'application/epub+zip'
 		};
 	}
+
+	/**
+	 * @throws \Exceptions\InvalidEbookFormatException
+	 */
+	public static function FromFilename(string $filename): self{
+		if(str_ends_with($filename, '.azw3')){
+			return self::Azw3;
+		}
+
+		if(str_ends_with($filename, '.kepub.epub')){
+			return self::Kepub;
+		}
+
+		if(str_ends_with($filename, '_advanced.epub')){
+			return self::AdvancedEpub;
+		}
+
+		if(str_ends_with($filename, '.epub')){
+			return self::Epub;
+		}
+
+		throw new \Exceptions\InvalidEbookFormatException();
+	}
 }