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.

backend/app/celery_app.py

@@ -130,6 +130,16 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.maintenance.prune_import_batches",
                 "schedule": 86400.0,
             },
+            # Audit 2026-06-02 — backfill_thumbnails's 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. Daily cadence is gentle
+            # (the task is idempotent and only enqueues regen for rows
+            # whose stored thumbnails are missing or corrupt).
+            "backfill-thumbnails-daily": {
+                "task": "backend.app.tasks.thumbnail.backfill_thumbnails",
+                "schedule": 86400.0,
+            },
         },
         timezone="UTC",
     )