fix: serve admin bundle as hashed immutable assets

The lazy-loaded admin CSS/JS resolved to bare runtime names served
no-store, the same scheme that left the main bundle vulnerable to stale
CSS in iOS WebViews after a deploy. The original reason for keeping them
bare (hashed admin files could 404 when nginx fronts aiohttp) no longer
holds: the backend image now carries the same deterministically hashed
assets nginx serves, and the App.svelte loader already falls back to the
bare name if a hashed asset ever 404s.

Resolve the admin assets through the same hashed/version-stable path as
the main bundle so they are emitted as immutable, cache-busting URLs.

Also drop the inert <meta http-equiv="Cache-Control/Pragma/Expires">
tags from the shell: browsers ignore http-equiv caching directives for
the document and use the real HTTP headers, which are already set.

backend/bot/app/web/templates/subscription_webapp.html

@@ -6,9 +6,6 @@
       name="viewport"
       content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no, viewport-fit=cover"
     />
-    <meta http-equiv="Cache-Control" content="no-store" />
-    <meta http-equiv="Pragma" content="no-cache" />
-    <meta http-equiv="Expires" content="0" />
     <meta name="robots" content="noindex, nofollow" />
     <meta name="theme-color" content="#03070b" />
     <link id="app-favicon" rel="icon" href="/favicon.ico" sizes="any" />