Include modification date

[nichtich]

The API results are dated. At the moment the dump is from 2022-06-30, so this date should be included in responses. See also #24 to increase update frequency.

[nichtich]

Could be part of #27.

[stefandesu]

Should the date be the import date? What if the imported dump is actually older than the import date?

[nichtich]

Modification date taken in this order:

• explicit date given to update script via command line script (e.g. --modified)
• else if import file given explicitly to import script (no import from stdin): use date of timestamp of import file
• else use current date

The date should best persitently be stored by the backend (in SQL database or in a file)

[stefandesu]

Now added in import script and API results.