-- Migration 142: Persistent thread key for reliable inbound case routing

ALTER TABLE email_messages
ADD COLUMN IF NOT EXISTS thread_key VARCHAR(500);

-- Backfill thread_key for existing rows.
-- Priority:
-- 1) First token from References (root message id)
-- 2) In-Reply-To
-- 3) Message-ID
UPDATE email_messages
SET thread_key = LOWER(
    REGEXP_REPLACE(
        COALESCE(
            NULLIF(
                SPLIT_PART(
                    REGEXP_REPLACE(COALESCE(email_references, ''), '^[\s<>,]+', ''),
                    ' ',
                    1
                ),
                ''
            ),
            NULLIF(in_reply_to, ''),
            NULLIF(message_id, '')
        ),
        '[<>\s]',
        '',
        'g'
    )
)
WHERE (thread_key IS NULL OR TRIM(thread_key) = '')
  AND (
        COALESCE(NULLIF(email_references, ''), NULLIF(in_reply_to, ''), NULLIF(message_id, '')) IS NOT NULL
      );

CREATE INDEX IF NOT EXISTS idx_email_messages_thread_key
ON email_messages(thread_key)
WHERE thread_key IS NOT NULL;

COMMENT ON COLUMN email_messages.thread_key IS 'Stable normalized thread key (root message-id/in-reply-to) for case routing';