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>

backend/app/services/scheduler_service.py

@@ -87,10 +87,15 @@ async def set_platform_cooldown(
     await session.execute(stmt)
 
 
-async def _platforms_in_cooldown(session: AsyncSession) -> dict[str, datetime]:
+async def active_platform_cooldowns(session: AsyncSession) -> dict[str, datetime]:
     """Return {platform: expires_at} for platforms whose cooldown is still
     in the future. Expired rows are ignored (a future maintenance sweep can
-    delete them; they don't affect routing decisions on their own)."""
+    delete them; they don't affect routing decisions on their own).
+
+    Exposed beyond scheduler_service so the manual check endpoint
+    (`/api/sources/<id>/check`) can defer bulk retries that would bowl
+    into the same rate limit the cooldown is preventing.
+    """
     rows = (await session.execute(
         select(AppSetting.key, AppSetting.value)
         .where(AppSetting.key.startswith(PLATFORM_COOLDOWN_KEY_PREFIX))
@@ -133,7 +138,7 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
         .order_by(Source.last_checked_at.asc().nulls_first(), Source.id)
     )).scalars().all()
 
-    cooldowns = await _platforms_in_cooldown(session)
+    cooldowns = await active_platform_cooldowns(session)
     settings = await ImportSettings.load(session)
 
     now = datetime.now(UTC)
@@ -212,7 +217,7 @@ async def scheduler_status(session: AsyncSession) -> dict:
         elif next_due_at is None or nca < next_due_at:
             next_due_at = nca
 
-    cooldowns = await _platforms_in_cooldown(session)
+    cooldowns = await active_platform_cooldowns(session)
 
     return {
         "last_tick_at": last_tick_at,