Remove separate vector database

[mzur]

The default database is now used as vector database, too.

This partially undos #670.

References biigle/maia#150

• Write a migration guide for the vector database (see here)
• Test the migration guide
• Update the biigle.de backup scripts https://github.com/biigle/biigle.de/pull/102
• Update the biigle.de database host setup instructions as well as the backup/restore instructions

[mzur]

Migration guide:

1. After step 4.1 of the updating instructions, do a vector database dump with the command:

    docker exec -i $(docker compose ps -q vector_database) \
        pg_dump --insert --on-conflict-do-nothing --data-only -U biigle -d biigle > biigle_vector_db.sql
   

2. After step 6.1, run:

   cat biigle_vector_db.sql | docker exec -i $(docker compose ps -q database) \
        psql -U biigle biigle -
   

3. Delete the vector_database Docker container and volume