Add RomVault ".dat" index format support

**is your feature request related to a problem? Please describe.**

Multiple projects like No-Intro, Myrient, Redump, MAME, TOSEC, ... use the ".dat"-file format from [RomVault](https://wiki.romvault.com/doku.php?id=introduction) and [DATVault](https://www.datvault.com/) for checksumming and integrity testing. This also allows people to build their collection with files from different sources while ensuring they've the same file.

List of indexing projects that use this format: https://wiki.romvault.com/doku.php?id=supported_dats

**Describe the idea / solution you'd like**

Would it be possible to implement support for these dat files in a way that allows:
* others to basically get a copy of the index (in this dat format) as if it was a regular file (e.g. in a /dats folder).
* to import/upload dat files to serve as index (manage the [DATRoot](https://wiki.romvault.com/doku.php?id=organizing_dats)) for files that should be there but aren't yet, e.g. if they're uploaded they will automatically appear under this file tree (aka be automatically sorted).
* to automatically update dat files in the background (when given via URL similar to how they're put into RomVault)
* to have the copyparty index stored in and/or synced to dat files.

Here is the link to the documentation for how RomVault does the scanning, and "fixing" (checking and moving files around): https://wiki.romvault.com/doku.php?id=scanning_fixing_levels

Edit: May help resolve #1395 as the dat format would allow to uniquely identify files and thereby make uploads deterministic (kinda).

Das Issue beschreibt die Integration der Unterstützung für RomVault-.dat-Dateien in copyparty zur Indexierung und automatischen Dateisortierung. Beginne damit, den vorhandenen Code für die Dateiindexierung und die Upload-Verarbeitung im Repository zu untersuchen, wahrscheinlich in den zentralen Modulen für die Dateiverwaltung. Lies die bereitgestellten Links zur RomVault-Dokumentation, um die Struktur des .dat-Formats und die Scan-Logik zu verstehen. Ermittle, wie .dat-Einträge auf den internen Dateibaum von copyparty abgebildet werden können, und implementiere Hintergrundaktualisierungen von URLs.

Vom Indexierungsmodell aus dem Issue-Text verfasst.