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## Problem It is not currently possible to disambiguate a timeline with an uninitialized PITR cutoff from one that was created within the PITR window -- both of these have `GcCutoffs::time == Lsn(0)`. For billing metrics, we need to disambiguate these to avoid accidentally billing the entire history when a tenant is initially loaded. Touches https://github.com/neondatabase/cloud/issues/28155. ## Summary of changes Make `GcCutoffs::time` an `Option<Lsn>`, and only set it to `Some` when initialized. A `pitr_interval` of 0 will yield `Some(last_record_lsn)`. This PR takes a conservative approach, and mostly retains the old behavior of consumers by using `unwrap_or_default()` to yield 0 when uninitialized, to avoid accidentally introducing bugs -- except in cases where there is high confidence that the change is beneficial (e.g. for the `pageserver_pitr_history_size` Prometheus metric and to return early during GC).