fix(audit-g5a): small architectural cleanups bundle

Five small G5 findings from the 2026-06-02 audit. Each is local and
follows an established FC pattern.

- download_service: replace hardcoded ('discord','pixiv') tuple with
  auth_type_for(platform) == 'token'. A 7th token-platform now picks
  up the right credential path without touching this site.

- /api/tags/<source_id>/merge enqueues recompute_centroid.delay after
  merge so the target's centroid reflects its new image set
  immediately. Daily list_drifted catches it within 24h, but eager
  recompute closes the suggestion-quality dip in the meantime.

- backfill_thumbnails added to beat_schedule (daily). The task
  docstring claimed periodic Beat but the entry was never registered,
  so the library got no self-healing thumbnail repair; only the
  manual admin-UI button fired it.

- modal.createAndAdd pushes a kind='fandom' tag into
  tagsStore.fandomCache so FandomPicker sees the new fandom on next
  open. Was: cache-gated load (length===0) skipped refetch, new
  fandom invisible until full page reload.

- cleanup cluster:
  - Drop .webp from cleanup_service.unlink — thumbnailer only writes
    .jpg/.png; the third tuple member was dead code.
  - Drop effective_date from /api/gallery/scroll response — no FE
    consumer reads it. Service still computes the attribute for
    timeline ordering; this just trims the JSON.
  - Rename store.recentMinute → store.recentRuns across the
    systemActivity store + three consumers (SystemActivitySummary,
    QueuesTable, SystemActivityTab). The data is the last 200 runs
    (not actually "last minute"), so the name lied.

NOT in this bundle: the duplicate tag-merge endpoint
(/api/tags vs /api/admin/tags) is harder — has 1 FE caller and 3 tests
on the admin variant; consolidation is its own change.

frontend/src/components/settings/SystemActivityTab.vue

@@ -14,7 +14,7 @@
         <QueuesTable
           :queues="store.queues"
           :workers="store.workers"
-          :recent-minute="store.recentMinute"
+          :recent-runs="store.recentRuns"
         />
       </v-card-text>
     </v-card>
@@ -202,7 +202,7 @@ function pollQueues() {
   if (document.hidden) return
   store.loadQueues()
   store.loadWorkers()
-  store.loadRecentMinute()
+  store.loadRecentRuns()
 }
 function pollFailures() {
   if (document.hidden) return