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/gallery_dl.py

@@ -163,6 +163,10 @@ class DownloadResult:
     run_stats: dict | None = None
     cursor: str | None = None
     posts_processed: int = 0
+    # Plan #708 B1 — the server's Retry-After seconds on a RATE_LIMITED result, so
+    # the platform cooldown matches the hint instead of a flat default. None when
+    # unknown (no header, or not a rate-limit failure).
+    retry_after_seconds: float | None = None
 
 
 def extract_errors_warnings(stderr: str) -> str: