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/stores/systemActivity.js

@@ -9,7 +9,7 @@ export const useSystemActivityStore = defineStore('systemActivity', () => {
   // Live polled state.
   const queues = ref(null)       // { queues: {name: depth|null}, fetched_at }
   const workers = ref(null)      // { workers: {hostname: {...}}, fetched_at }
-  const recentMinute = ref([])   // last-60s rows (for Overview summary)
+  const recentRuns = ref([])   // last-60s rows (for Overview summary)
   const failures = ref(null)     // { recent, count_by_type, since }
 
   // Paginated runs (Activity tab "All recent activity" pane).
@@ -45,14 +45,14 @@ export const useSystemActivityStore = defineStore('systemActivity', () => {
     }
   }
 
-  async function loadRecentMinute() {
+  async function loadRecentRuns() {
     // Used by the Overview summary card: pull last 60s of runs to compute
     // per-queue ok/err counts. One call covers all queues; UI groups.
     try {
       const body = await api.get('/api/system/activity/runs', {
         params: { limit: 200 },
       })
-      recentMinute.value = body.runs || []
+      recentRuns.value = body.runs || []
     } catch (e) {
       lastError.value = e.message
     }
@@ -106,10 +106,10 @@ export const useSystemActivityStore = defineStore('systemActivity', () => {
   }
 
   return {
-    queues, workers, recentMinute, failures, summary,
+    queues, workers, recentRuns, failures, summary,
     runs, runsCursor, runsHasMore, runsFilter,
     loading, lastError,
-    loadQueues, loadWorkers, loadRecentMinute,
+    loadQueues, loadWorkers, loadRecentRuns,
     loadRuns, loadFailures, loadSummary, setFilter,
   }
 })