* feat(db): background_state table and the (hop_count, iata) route index
* perf(db): one /routes statement per filter shape, hop pair on custom plans
* feat(db): keyset route retention and grace slices at exact cutoffs
* feat(background): hourly route retention with a bounded grace walk and watermark
* fix(background): bound the grace walk at microsecond cutoffs
* feat(db): stamp-free reconfirm sweep over the route primary key
* feat(background): pace route reconfirm at routes.reconfirm_rows_per_hour, no stamps
* test(routes): drive route retention end to end and tidy comments
* fix(profiling): sample during the hourly route retention run
