fix: garbage collection with more sensible limits grouped by plan/repo

[garethgeorge]

Garbage collection now enforces limits that better guarantee correctness w.r.t. new scheduling features (e.g. last run relative scheduling) and that make more sense in the UI.

For each repo

• Prune and Check operations retain the last 12 executions. More than this should not be necessary. They are retained for a maximum time of 12 months.
• Stats operations are retained for up to 1 year OR 100 data points to preserve preformance.
• All other operations are retained up to 30 days or 100 operations.

Note that "snapshot" association overrides these rules e.g. if an operation is associated with a snapshot that is still present in the repo, the operation will not be deleted until that snapshot is forgotten.