fix(download): align gallery-dl subprocess timeout to Celery soft limit

SourceConfig.timeout defaulted to 3600s (1 hour), but download_source's
Celery task has soft_time_limit=900s and hard time_limit=1200s. So
gallery-dl never hit its own subprocess.run timeout — Celery always
killed it first. The hard SIGKILL leaves no terminal flip on the
DownloadEvent, which then sat pending/running until the recovery sweep
flipped it to error at 30 min from start. From the operator's seat that
was a ~30–40 min "hang" on every retry of a broken source.

Pinning the default to 900s (matching Celery's soft_time_limit) lets
subprocess.run raise TimeoutExpired cleanly inside Celery's window, and
the existing `except subprocess.TimeoutExpired` branch
(gallery_dl.py:635) captures it as a clean error_type='timeout' with a
real message. The DownloadEvent flips to error in ~15 min instead of
waiting on the recovery sweep at 30.

Per-source bumps still live in source.config_overrides for legitimately
long first-syncs. The new _DEFAULT_GDL_TIMEOUT_SECONDS constant carries
the rationale and the Celery-soft-limit dependency in its comment.

Operator-flagged 2026-05-30 on 59-source strand pile retries.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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),
         )