feat: Admin is a left rail with how many wait on you in each queue, and the Overview opens with a verdict, the queues that need you and since when, then the clear ones on one line (M503 #5605)

- GET /api/admin/summary: every admin queue (requests, reported tracks,
  playback errors, duplicates, suspect sources, missing files) with its
  count and the age of its oldest item, in one query.
- The admin layout drops the 11-tab strip for SectionRail, as Settings and
  the Collection have, with each queue's count as its badge; the counts
  refresh on each admin navigation and every 30s.
- The Overview leads with "Nothing needs you." or "N things need you.",
  lists each waiting queue with "oldest 3d ago" and a link to it, counts a
  failed library scan as a problem, and names the empty queues on one line.
  The request and quarantine previews and the stat tiles are gone; those
  queues are worked on their own pages. A failed summary says so with Try
  again instead of reading as all-clear.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/components/TabStrip.svelte

@@ -1,7 +1,7 @@
 <script lang="ts" generics="Id extends string">
   // In-page tabs: one strip of buttons switching what a single page shows
-  // (#5416). The same look as AdminTabs and DiscoverTabs, which are route
-  // navigation and so stay links; this is for state inside one page, such as
+  // (#5416). The same look as DiscoverTabs, which is route
+  // navigation and so stays links; this is for state inside one page, such as
   // Liked's Artists | Albums | Tracks or a queue's status filter.
   type Item = {
     id: Id;