An as-run log the operator can trust

[robert-dean]

Split from #32. Persistence of the rundown (station_lineup) and every break writer that #32 listed as missing have since been built; this is what is left.

The running order says what was planned. Nothing records what actually aired, in order, with times the operator can trust: there is no as-run table or migration. play_history and the activity feed each hold part of the answer, and neither is a record of breaks, records and silences as they went out.

#32's note on corroborating the playhead is advice for whoever builds this rather than a separate task.

[robert-dean]

Closing: built in #765.

• deadair.as_run (migration 0113) gets one row per item on the rundown's onAired edge, records, breaks and programmes alike. The row before it is closed in the same breath, and a stand-down closes the open one. An end is only ever observed: a row left open behind the item on air means the station stopped without a stand-down.
• The two writes land from separate scopes and are safe in either order. That was checked against a real database with the writes deliberately reversed.
• GET /history/as-run is a keyset page over the log unioned with the gap, air.on and air.off events station_events already holds, read rather than copied. until jumps to a moment.
• Check-up has an As run tab.
• Kept 400 days. The gap and air lines reach back as far as the activity feed is kept.

#32's playhead advice did not apply: nothing here schedules against the playhead, it only records the player's own confirmation.