feat(downloads): platform cooldown honors server Retry-After — B1 (plan #708)

Owning the native client means we see the 429 Retry-After header — previously
discarded. PatreonAPIError now carries `retry_after`; on a PERSISTENT page-fetch
429 the client attaches the server's raw Retry-After seconds. New
DownloadResult.retry_after_seconds; patreon_ingester._failure_result sets it on
RATE_LIMITED. download_service._update_source_health passes it to
set_platform_cooldown as `seconds=`, clamped to [60, 3600] (a tiny hint can't
leave the platform effectively un-cooled; a huge one can't strand it for hours);
no hint → the flat 900s default. So a rate-limited platform cools for as long as
the server actually asks, not a fixed guess.

Tests: terminal 429 surfaces retry_after (test_patreon_client); cooldown honors
+ clamps the hint, falls back to default when absent (test_download_service).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/patreon_ingester.py

@@ -162,10 +162,14 @@ class PatreonIngester(Ingester):
             else:
                 error_type = ErrorType.NETWORK_ERROR
         log.warning("Patreon ingest failed (%s): %s", error_type.value, message)
-        return _result(
+        result = _result(
             success=False, return_code=1,
             error_type=error_type, error_message=message,
         )
+        # plan #708 B1: carry the server's Retry-After up to the cooldown.
+        if error_type == ErrorType.RATE_LIMITED:
+            result.retry_after_seconds = getattr(exc, "retry_after", None)
+        return result
 
 
 async def verify_patreon_credential(