Add keep_open_until to /allowed for hold-door; fix 9 bugs

/allowed returns keep_open_until from the newest approved hold in
doorlog; /longpoll skips hold events to avoid spurious open pulses.

Fixes: assert->raise for SECRET check, text() on 403, remove dead
/logs code, flatten auth decorator, by_slackid None fallback, load
kube config once, guard missing slack command, backoff on PyMongoError,
mongo->mongosh.

mongo-init.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-mongo <<EOF
+mongosh --quiet <<EOF
 rs.initiate({
     _id: 'rs0',
     members: [