Improve backup file naming in Synology DSM backup agent

[mib1185]

Proposed change

This will include the name and the timestamp of the backup into the backup file names. Backups taken during beta of 2025.2 might not be readable anymore, but I do not consider this as a breaking change, since it is the beta phase 😬

resulting files on the NAS:

• needs rebase after Don't blow up when a backup doesn't exist on Synology DSM #136913

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[home-assistant]

Hey there @hacf-fr, @Quentame, mind taking a look at this pull request as it has been labeled with an integration (synology_dsm) you are listed as a code owner for? Thanks!

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