Add support for serving under prefix

[glensc]

Describe the bug

Add support, so instead of occupying whole domain.example.com, would be able to serve under /your_spotify path (for example).

Expected behavior

The intention would be to be able to serve under swag:

• https://docs.linuxserver.io/general/swag/

And to have an example config published:

• https://docs.linuxserver.io/general/swag/#preset-proxy-confs

Currently, there are published only your-spotify.subdomain.conf and your-spotify-api.subdomain.conf:

• https://github.com/linuxserver/reverse-proxy-confs/blob/88eb33a556abb6f014276cd05cda9ee774c56148/your-spotify.subdomain.conf.sample
• https://github.com/linuxserver/reverse-proxy-confs/blob/88eb33a556abb6f014276cd05cda9ee774c56148/your-spotify-api.subdomain.conf.sample

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I guess the starting point would be to agree on the variable,

• https://github.com/Yooooomi/your_spotify/blob/b205bfb606fbc72f41eb79fb6fcb9a230bf8819d/server/src/tools/env.ts

perhaps APP_PREFIX: z.string().optional()