fix(strava): resume an unfinished backfill where the last run stopped

One run reads at most forty pages of a hundred activities. A longer history
ended as incomplete with no note of how far it got, so every boot read the
same newest pages again, spent up to 190 of the instance's 200 requests per
fifteen minutes on them, and never reached the older activities.

The run now records the start of the oldest activity it read and sends the
next run for after the rate window; that run asks Strava for the activities
before it. A run that gets no further records nothing and sends nothing. The
place is cleared when the backfill completes.

src/lib/export/backup-plan.ts

@@ -960,6 +960,7 @@ export const USER_COLUMN_BACKUP_CLASS = {
   coachLastSeenAt: "OPERATIONAL",
   stravaLastActivityAt: "OPERATIONAL",
   stravaBackfillCompletedAt: "OPERATIONAL",
+  stravaBackfillResumeBefore: "OPERATIONAL",
   healthKitLastSyncedAt: "OPERATIONAL",
   healthKitLastSyncTrigger: "OPERATIONAL",
   healthKitLastBackgroundSyncAt: "OPERATIONAL",