Improve V1 compatibility

[hbruch]

This PR makes some minor adaptations to prepare the api.parkendd.de server switch:

• if total is unknown for a lot, the v1 api returns 0, as total is mandatory
• if last_updated is unknown, we fall back to last_downloaded, as last_updated is mandatory
• root (/) is redirected to api/, so current clients accessing api.parkendd.de will not break
• /<city> requests are redirected to /api/<city>, so current clients accessing api.parkendd.de will not break
• the page stats which is currently served at / is moved to /stats/
• if no source_url is declared, public_url is returned, as source_url is mandatory.
• free is only returned if known (Note: while this is conform to the schema, current v1 seems to return 0 if no data is availble, so this might break clients)