-- AP2.2f / ADP Backlog Epic-Hierarchie — parent_backlog_id + item_kind epic

ALTER TABLE backlog_items
    ADD COLUMN IF NOT EXISTS parent_backlog_id UUID NULL
        REFERENCES backlog_items (id) ON DELETE SET NULL;

CREATE INDEX IF NOT EXISTS idx_backlog_parent_backlog
    ON backlog_items (tenant_id, initiative_id, parent_backlog_id)
    WHERE parent_backlog_id IS NOT NULL;

ALTER TABLE backlog_items DROP CONSTRAINT IF EXISTS backlog_items_item_kind_check;
ALTER TABLE backlog_items
    ADD CONSTRAINT backlog_items_item_kind_check
        CHECK (item_kind IN ('story', 'bug', 'issue', 'epic'));