Add Database Backup Feature (Disabled by Default) and Use postgres User for Multi-DB Support

[NUZAT-TABASSUM]

New features:

• The backup feature is disabled by default. To enable it, set database_backup_enabled: true and provide a value for database_backup_output_path.

• You can set the backup path, schedule, how many old backups to keep, file ownership, and more—all through variables.

• Previously, backups used the opencast database user. Now, the role uses the postgres user by default for backups. This makes it much easier to back up multiple databases (like opencast, tobira, etc.) with a single setup, since postgres can access all databases by default. No need to create extra users or adjust configs for each DB.

• Just list the databases you want to back up in the database_backup_dbs variable, for example:

  database_backup_dbs:
    - opencast
    - tobira
    - mycustomdb

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