Add support for transliteration of German "umlaute" in slugs

**Is your feature request related to a problem? Please describe.**
If the title contains a German umlaut (`äöü`) and use `clean_accents: true`, they are converted to plain letters `aou`. While this is suitable for actual accents (e.g. `é`), it looks really odd for umlaute.

**Describe the solution you'd like**
In German there is an official rule for transliterating special characters:
```
ä -> ae
ö -> oe
ü -> ue
ß -> ss
```
The last rule is already covered by `clean_accents: true`. It would be nice to change the conversion of the remaining characters or provide an additional setting to enable it.

**Describe alternatives you've considered**
Manually editing the file name is possible but inconvenient. The ability to edit slugs as discussed in #445 would make this less of an issue.

**Additional context**
N/A

Start by locating the slug-generation path and the handling of the clean_accents setting. Compare the current conversion with the requested ä→ae, ö→oe, ü→ue, and existing ß→ss behavior, while reviewing issue #445 for related slug-editing context. Done means the chosen behavior is implemented and verified for German umlauts without regressing other accent conversion.

Written by the indexing model from the issue text.