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/QueuesTable.vue

@@ -35,7 +35,7 @@ import { computed } from 'vue'
 const props = defineProps({
   queues: { type: Object, default: null },     // store.queues
   workers: { type: Object, default: null },    // store.workers
-  recentMinute: { type: Array, default: () => [] }, // store.recentMinute
+  recentRuns: { type: Array, default: () => [] }, // store.recentRuns
   compact: { type: Boolean, default: false },
 })
 
@@ -80,7 +80,7 @@ function activeCount(name) {
 
 const recentByQueue = computed(() => {
   const out = {}
-  for (const r of props.recentMinute) {
+  for (const r of props.recentRuns) {
     if (!out[r.queue]) out[r.queue] = { ok: 0, err: 0 }
     if (r.status === 'ok') out[r.queue].ok++
     else if (r.status === 'error' || r.status === 'timeout') out[r.queue].err++

frontend/src/components/settings/SystemActivitySummary.vue

@@ -24,7 +24,7 @@
       <QueuesTable
         :queues="store.queues"
         :workers="store.workers"
-        :recent-minute="store.recentMinute"
+        :recent-runs="store.recentRuns"
         compact
       />
     </v-card-text>
@@ -47,7 +47,7 @@ function pollOnce() {
   if (document.hidden) return
   store.loadQueues()
   store.loadWorkers()
-  store.loadRecentMinute()
+  store.loadRecentRuns()
 }
 
 onMounted(() => {

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