feat(downloads): bulk retry respects cooldown; single-source RETRY overrides

Today's platform-cooldown commit (61ce1ce) only filtered the scan tick
— manual /api/sources/<id>/check still bypassed it. Operator-flagged
2026-05-30: clicked "Retry failed" on a Patreon failure pile and saw
every one go 'queued' without realising the cooldown wasn't in the
loop. Bulk retry with N sources on a cooled-down platform bowls right
back into the rate limit the cooldown is trying to prevent.

**Backend (`/api/sources/<id>/check`):**
- Reads optional `?force=true` query flag.
- Without force: queries `active_platform_cooldowns` (renamed from the
  private `_platforms_in_cooldown` since it's now a cross-module API).
  If the source's platform is in cooldown, returns **202** with
  `{status: 'deferred', platform, cooldown_until}` — no event created,
  no dispatch.
- With force: cooldown skipped entirely.
- In-flight guard always applies (no point creating duplicate pendings).

**Frontend (`sourcesStore.checkNow(id, {force=false})`):** new optional
`force` flag → adds `?force=true` to the URL.

**Frontend (`DownloadsTab`):**
- `onRetrySource` (single-source RETRY click): passes `force: true` →
  explicit operator override, useful for rapid auth-fix testing.
- `onRetryAll` (RETRY ALL + MaintenanceMenu "Retry failed"): no force →
  cooldown respected. Tallies `deferred` alongside `queued` /
  `already_running`; toast reads e.g. *"5 queued, 12 deferred
  (cooldown), 3 already running"*. That count is the operator's
  diagnostic answer for "is rate-limit the cause of most failures?"
  (12-of-20 deferred → yes; 0 deferred → no).

**Auto-resume:** no new sweep needed. Deferred sources still have stale
`last_checked_at`, so the next scan tick after the cooldown AppSetting
expires picks them up via `select_due_sources` (which already filters
on `active_platform_cooldowns`).

Tests: two new — deferred-on-cooldown returns 202 with the right body
and no dispatch; force=true overrides the cooldown and creates the
event normally.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/sources.js

@@ -67,10 +67,17 @@ export const useSourcesStore = defineStore('sources', () => {
   // FC-3c: trigger a download for one source. Returns {download_event_id,status}.
   const checkingIds = ref(new Set())
 
-  async function checkNow(id) {
+  // force=true bypasses the platform-rate-limit cooldown gate. Single-
+  // source RETRY clicks pass it (operator-explicit override, useful for
+  // rapid auth-fix testing); bulk RETRY ALL / MaintenanceMenu retries
+  // leave it off so the cooldown does its preventive job.
+  async function checkNow(id, { force = false } = {}) {
     checkingIds.value = new Set(checkingIds.value).add(id)
     try {
-      return await api.post(`/api/sources/${id}/check`)
+      const url = force
+        ? `/api/sources/${id}/check?force=true`
+        : `/api/sources/${id}/check`
+      return await api.post(url)
     } finally {
       const next = new Set(checkingIds.value)
       next.delete(id)