feat(inbox): scope all inbox queries by workspace_id

Inbox items were previously queried only by recipient, which leaked data
across workspaces. All list/count/batch operations now filter by
workspace_id from the X-Workspace-ID header.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/cmd/server/notification_listeners_test.go

@@ -18,6 +18,7 @@ import (
 func inboxItemsForRecipient(t *testing.T, queries *db.Queries, recipientID string) []db.ListInboxItemsRow {
 	t.Helper()
 	items, err := queries.ListInboxItems(context.Background(), db.ListInboxItemsParams{
+		WorkspaceID:   util.ParseUUID(testWorkspaceID),
 		RecipientType: "member",
 		RecipientID:   util.ParseUUID(recipientID),
 		Limit:         100,