feat(downloads): return structured 429 rate-limit responses with retry_after

On a CivitAI/CivArchive 429, download-model and download-model-get now
return HTTP 429 with {"reason": "rate_limited", "retry_after": N}
instead of a generic 500 string, and the queue row goes back to
"queued" rather than history as failed — so queue drivers can
auto-pause and retry later instead of burning through the queue.

- new DownloadRateLimitError carrying retry_after/host (opt-in via
  raise_on_rate_limit on Downloader; other call sites keep the legacy
  string behavior)
- fail-fast pre-flight gate in DownloadManager consults
  RateLimitCoordinator before acquiring the semaphore slot: hosts in
  cooldown get an immediate structured 429, no HTTP request attempted
- best-effort 429 detection for the aria2 backend

py/routes/handlers/model_handlers.py

@@ -1761,7 +1761,8 @@ class ModelDownloadHandler:
             payload = await request.json()
             result = await self._download_use_case.execute(payload)
             if not result.get("success", False):
-                return web.json_response(result, status=500)
+                status = 429 if result.get("reason") == "rate_limited" else 500
+                return web.json_response(result, status=status)
             return web.json_response(result)
         except DownloadModelValidationError as exc:
             return web.json_response({"success": False, "error": str(exc)}, status=400)
@@ -1819,7 +1820,8 @@ class ModelDownloadHandler:
             mock_request = type("MockRequest", (), {"json": lambda self=None: future})()
             result = await self._download_use_case.execute(data)
             if not result.get("success", False):
-                return web.json_response(result, status=500)
+                status = 429 if result.get("reason") == "rate_limited" else 500
+                return web.json_response(result, status=status)
             return web.json_response(result)
         except DownloadModelValidationError as exc:
             return web.json_response({"success": False, "error": str(exc)}, status=400)