Add per-user item ownership and isolation

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.

internal/models/models.go

@@ -30,6 +30,7 @@ type User struct {
 
 type Item struct {
 	ID                  int64
+	UserID              int64
 	Name                string
 	SearchQuery         string
 	URL                 string