Setting default snapshot retention policy

### Epic

https://github.com/flowforge/flowforge/issues/2421

### Description

As a FlowForge User,

I want to have the possibility to set a default retention policy,

So that I not save endless snapshots.

### Which customers would this be availble to

All Users, (CE)

### Acceptance Criteria

- [ ] Should provide an option for users to set a default retention policy for their snapshots.
- [ ] When the limit is reached, the system should automatically delete the oldest snapshot.
- [ ] Users should have the ability to mark certain manual snapshots as 'do not delete' so that they are excluded from the retention policy.

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start by reading the linked epic, issue 2421, and trace where snapshot settings and snapshot deletion are handled. The work is complete when users can configure a default retention policy, oldest snapshots are removed at the limit, and marked manual snapshots are preserved.

Written by the indexing model from the issue text.