Adds two managed system overlays (l4d2center-maps, cedapug-maps) that
fetch curated map archives from upstream sources and reconcile addons
symlinks for non-Steam maps. A daily systemd timer enqueues a coalesced
refresh_global_overlays worker job; downloads, extraction, and rebuilds
run in the existing job worker and surface in the job log UI.
Schema: GlobalOverlaySource / GlobalOverlayItem / GlobalOverlayItemFile
plus nullable Job.user_id so system jobs render as "system" in the UI.
The new builder reconciles symlinks against the per-source vpk cache
and leaves foreign symlinks untouched. Initialize-time guard refuses
to mount a partial overlay if any expected vpk is missing from cache.
Refresh service uses shutil.move to handle EXDEV when /tmp and the
cache live on different filesystems.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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