feat: Allow MongoDB backup of all databases (omit database name)

Feature Request

Problem

Currently, when creating a MongoDB backup in Dokploy, the database name field is required. This means users must create a separate backup entry for each database in their MongoDB instance. There is no way to back up all databases at once.

Proposed Solution

Allow the database name field to be left empty for MongoDB backups. When empty, mongodump runs without the -d flag, which causes it to dump all databases in the instance.

This is a common use case — users often want a full instance backup rather than individual database backups.

Changes Required

1. getMongoBackupCommand — Conditionally include the -d flag only when a database name is provided
2. Schema validation — Allow empty database string specifically for MongoDB (other database types still require it)
3. UI — Add a hint on the database field for MongoDB: "Leave empty to back up all databases"

Additional Context

• mongodump natively supports this — omitting -d dumps all databases
• This only affects MongoDB; other database types (Postgres, MySQL, MariaDB) still require a database name
• No breaking changes — existing backups with a database name continue to work as before

Start at the getMongoBackupCommand entry point and trace the MongoDB backup schema and database-field UI described in the issue. The change is done when an empty MongoDB database value omits -d, shows the requested hint, and other database types still require a name.