Adds Overlay.type and Overlay.user_id with two partial unique indexes
(externals globally unique by name; user overlays unique per user).
Adds WorkshopItem registry keyed on steam_id and a pure many-to-many
overlay_workshop_items association. Adds Job.overlay_id for build_overlay
job tracking. Switches overlays.id to AUTOINCREMENT so deleted IDs are
never reused.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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