Document the api layer's operational models, and fix two headers that had gone stale against the code

Plugin/unraid/api/fallback.php

@@ -5,6 +5,13 @@
 //   state, tier activation, handback strikes, covered container status — for the fallback
 //   tab's 30s poll.
 //
+// OPERATIONAL MODEL
+//   Computed fresh on every request, deliberately uncached. The inputs are small local state
+//   files that fallback.sh rewrites as it moves between states, so assembling them costs about
+//   nothing — and a cached fallback picture is the one kind of stale this tab must never serve.
+//   A page showing NORMAL because the answer was cached before the switch is worse than a page
+//   that took an extra moment to load.
+//
 // DESIGN PRINCIPLES
 //   Thin transport. Every judgement about what a state file means lives in
 //   include/fallback.php. This file exists to give the browser a URL.