[FreshRSS] Migrate MariaDB data to existing PostgreSQL database

[letwir]

Depends on LXC 240 I/O load returning to normal.

Tasks:

• Back up FreshRSS data, extensions, and MariaDB.
• Use FreshRSS cli/db-backup.php to create portable per-user SQLite backups.
• Configure the existing PostgreSQL FreshRSS database.
• Change FreshRSS DB type from mysql to pgsql.
• Restore with cli/db-restore.php.
• Verify Web UI, API, feed count, unread state, and cron refresh.
• Keep MariaDB intact until validation passes.