Delete unused snapshots on workorders retention cleanup

[jyeshe]

Description

This PR adds an deletion for unused snapshots (those are referenced by no work orders anymore).

There are two scenarios when snapshots needs to be deleted:
a) on project deletion, when workflows deletion takes care of it
b) on data retention cleanup, when much work orders are left after deleting the old ones.

The filtering of the snapshots to delete come from the set difference between snapshots from the deleted work orders and snapshots that are still in use (referenced by recent work orders that are still in the retention period).

Closes #1832

Validation steps

0. Setup some retention period.
1. Insert 10k work orders, stop Lightning and update their date to be out of the retention period (older). Also update the state of both the work orders and runs to success.
2. Take note of the snapshot id used.
3. Edit the workflow.
4. Insert 2k work orders and update their date to be out of the retention period (older).
5. Insert more 2k work orders.
6. Call Lightning.Projects.perform(%Oban.Job{args: %{"project_id" => "4adf2644-ed4e-4f97-a24c-ab35b3cb1efa", "type" => "data_retention"}}) using your project_id
7. It shall delete all work orders from steps 1 and 4 in less than 3 seconds. The snapshot of step 2 is also deleted.

and only one snapshot is left

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[jyeshe]

Unrelated flaky:

1. test Allow low priority access users to retry steps and create workorders Users with low priority access to the workflow canvas will automatically be locked in a snapshot when the high prior uses saves the workflow (LightningWeb.WorkflowLive.EditTest)
   test/lightning_web/live/workflow_live/edit_test.exs:3654
   Expected false or nil, last got true
   code: refute_eventually(low_priority_view |> has_element?("#inspector-workflow-version", "latest"))
   stacktrace:
   test/lightning_web/live/workflow_live/edit_test.exs:3693: (test)

[midigofrank]

Nicely done @jyeshe

[stuartc]

@jyeshe this is great! Thanks! I just merged main in, and did a small (uneventful) change for the project_files test, changing all the DateTime.utc_now() to Lightning.current_time(), it doesn't affect the tests at all but nice to default to since we can freeze/stub that call.