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.

app/slack.py

@@ -1,3 +1,4 @@
+import asyncio
 import os
 from datetime import datetime, timezone
 from typing import Tuple
@@ -70,6 +71,7 @@ async def slack_log_fwd(app):
 
         except PyMongoError as e:
             print(e)
+            await asyncio.sleep(5)
 
 # -> approved, user, err
 def slack_authz(authGroup: str, slackId: str, channel_id: str) -> Tuple[bool, str, str]:
@@ -96,6 +98,10 @@ async def slack_open(request):
         return "Invalid token (are you Slack?)", 401
 
     command = request.form.get("command")
+    if not command:
+        print("WARN: /slack-open route accessed without command")
+        return text("Missing command", status=400)
+
     door = command.removeprefix("/open-").replace("-", "")
 
     authGroup = fauthGroup(door)
@@ -110,7 +116,7 @@ async def slack_open(request):
         request.form.get("channel_id"),
     )
 
-    userExtra = f"{request.form.get("user_id")} (slack u/n: {request.form.get("user_name")})" # slackName can be changed by user
+    userExtra = f"{request.form.get('user_id')} (slack u/n: {request.form.get('user_name')})" # slackName can be changed by user
 
     doors = [door]
     if door == "alldoors":