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/handlers/handlers.go

@@ -190,6 +190,15 @@ func (a *App) page(r *http.Request, title, active string) templates.Page {
 	}
 }
 
+// userID returns the signed-in user's id. Handlers in this group all run under
+// RequireAuth, so a user is always present; 0 is only returned defensively.
+func (a *App) userID(r *http.Request) int64 {
+	if u := auth.CurrentUserFromRequest(r); u != nil {
+		return u.ID
+	}
+	return 0
+}
+
 // noListFS wraps an http.FileSystem and refuses to open directories, which
 // stops http.FileServer from emitting an auto-generated directory listing.
 type noListFS struct{ fs http.FileSystem }