diff --git a/backend/app/services/gallery_dl.py b/backend/app/services/gallery_dl.py
index e149477..2883bd4 100644
--- a/backend/app/services/gallery_dl.py
+++ b/backend/app/services/gallery_dl.py
@@ -42,6 +42,15 @@ class ErrorType(StrEnum):
     UNKNOWN_ERROR = "unknown_error"
 
 
+# Pinned to download_source's Celery soft_time_limit (900s, see
+# tasks/download.py:32). Anything larger and Celery kills the task before
+# subprocess.run can raise TimeoutExpired — leaving the DownloadEvent
+# stranded for the recovery sweep instead of capturing a clean timeout
+# error. Per-source bumps live in source.config_overrides for legitimately
+# long syncs. Operator-confirmed 2026-05-30 (~40-min hang investigation).
+_DEFAULT_GDL_TIMEOUT_SECONDS = 900
+
+
 @dataclass
 class SourceConfig:
     content_types: list[str] = field(default_factory=lambda: ["all"])
@@ -51,7 +60,7 @@ class SourceConfig:
     filename_pattern: str | None = None
     skip_existing: bool = True
     save_metadata: bool = True
-    timeout: int = 3600
+    timeout: int = _DEFAULT_GDL_TIMEOUT_SECONDS
 
     @classmethod
     def from_dict(cls, data: dict) -> SourceConfig:
@@ -63,7 +72,7 @@ class SourceConfig:
             filename_pattern=data.get("filename_pattern"),
             skip_existing=data.get("skip_existing", True),
             save_metadata=data.get("save_metadata", True),
-            timeout=data.get("timeout", 3600),
+            timeout=data.get("timeout", _DEFAULT_GDL_TIMEOUT_SECONDS),
         )