Items are now private to their owner. Add items.user_id (migrated in
place, existing items backfilled to the first admin) and scope every
item, results, and dashboard view to the signed-in user via owner-scoped
queries plus an ownedItem 404 guard. Admins get strict isolation too;
elevation stays limited to settings and user management.
Alert routing follows ownership: deal emails go only to the item owner
and the weekly digest is built per-recipient from their own items. The
scheduler still polls every active item; the Apify/eBay budget stays a
shared pool visible to all.
Add TestItemsArePrivatePerUser and seed owners in db tests.
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