-- Migration: Add document_group_id and page_number for multi-page document grouping.
-- A document_group_id groups multiple sessions that belong to the same scanned document.
-- page_number is the 1-based page index within the group.

ALTER TABLE ocr_pipeline_sessions
    ADD COLUMN IF NOT EXISTS document_group_id UUID,
    ADD COLUMN IF NOT EXISTS page_number INT;

-- Index for efficient group lookups
CREATE INDEX IF NOT EXISTS idx_ocr_sessions_document_group
    ON ocr_pipeline_sessions (document_group_id)
    WHERE document_group_id IS NOT NULL;