Refactor repo deletion to move db/provider logic behind interface

[dmjb]

Relates to #2845

Change the interface of the repo service so that it takes care of DB queries and provider instantiation. This moves various details about repo cleanup out of the controlplane.

As part of this, change the provider cleanup process to delete webhooks directly instead of deleting repos. This avoided a circular dependency, and more closely aligns with the idea that webhook management is a provider-specific detail.

Tested locally.

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[coveralls]

coverage: 50.589% (+0.07%) from 50.517%
when pulling a559df5 on delete-repo-refactor
into e74fc00 on main.