Support revision retention policies and permanent deletion of historical revisions

[IceAndSnow]

Is your feature request related to a problem? Please describe.
Gramps Web keeps historical revisions in undo.db. This is useful for undo functionality, but makes it difficult to fully remove data when needed (e.g. privacy/GDPR requirements or accidental sensitive entries).

Right now, cleaning up revision history requires manual database access.

Describe the solution you'd like
It would be useful to support revision lifecycle management in the API (could be restricted to only be allowed for admins):

• Retention policy configuration
  • Allow setting a retention period for revisions (e.g. 7/30/90 days or unlimited).
  • Automatically purge revisions older than the configured period.
• Manual revision deletion
  • API support to delete:
    • a specific revision
    • all revisions for an object
    • all revisions in a tree

This keeps recent undo capabilities while ensuring older or sensitive history can be fully removed.

Describe alternatives you've considered

• Directly editing/purging undo.db (works but requires manual cron-job configuration or similar when setting up Gramps)
• Disabling revision history (loses undo safety)
• Keeping all revisions indefinitely (not suitable for privacy-sensitive use cases)

Additional context
Eventually exposing this functionality in the UI would be nice

[DavidMStraub]

Clarification for people randomly bumping into this issue:

This is not a privacy or GDPR violation bug in Gramps Web. Revision history is an intentional feature, and it is only accessible to users that are explicitly granted edit rights, which includes rights to view all confidential data in the tree.

That being said, the feature request is valid and there are additional reasons why one would want this (e.g. database bloat after imports/bulk deletes).

One general challenge is that Gramps Web API currently doesn't support cron jobs, so retention policies would have to hook into other requests, which is not ideal.

Manual deletion is very easy to implement.