feat(admin): add never-subscribed broadcast target with audience counts

Add a broadcast audience for users who registered but never had any subscription or trial (no Subscription rows at all), backed by a new get_user_ids_without_any_subscription DAL helper and a 'never' target in the webapp broadcast route.

Add GET /api/admin/broadcast/audience-counts so the audience dropdown shows the recipient count next to each option, with graceful fallback when counts are unavailable.

frontend/src/admin/sections/BroadcastSection.svelte

@@ -1,16 +1,27 @@
 <script>
   import { Textarea } from "$components/ui/index.js";
   import { Send } from "$components/ui/icons.js";
-  import { getContext } from "svelte";
+  import { getContext, onMount } from "svelte";
   import { Label } from "$components/ui/primitives.js";
   import { AdminButton, AdminSelect } from "$components/patterns/admin/index.js";
 
   export let at;
   const broadcastStore = getContext("broadcastStore");
 
-  $: ({ broadcastTarget, broadcastText, broadcastBusy, broadcastResult } = $broadcastStore);
+  $: ({ broadcastTarget, broadcastText, broadcastBusy, broadcastResult, broadcastCounts } =
+    $broadcastStore);
 
   const BROADCAST_TARGET_OPTIONS = broadcastStore.BROADCAST_TARGET_OPTIONS;
+
+  // Append the resolved audience size to each option once counts are loaded.
+  $: targetOptions = BROADCAST_TARGET_OPTIONS.map((option) => {
+    const count = broadcastCounts?.[option.value];
+    return count == null ? option : { ...option, label: `${option.label} (${count})` };
+  });
+
+  onMount(() => {
+    broadcastStore.loadCounts();
+  });
 </script>
 
 <div class="admin-card">
@@ -24,7 +35,7 @@
         <span>{at("broadcast_label_audience", {}, "Аудитория")}</span>
         <AdminSelect
           value={broadcastTarget}
-          items={BROADCAST_TARGET_OPTIONS}
+          items={targetOptions}
           ariaLabel={at("broadcast_label_audience", {}, "Аудитория")}
           onValueChange={(value) => broadcastStore.updateField({ broadcastTarget: value })}
         />