-- Migration 002: add per-inbox delivery sequence numbers.
--
-- Adds a `seq` column to the deliveries table and a separate counter table
-- that tracks the next sequence number per (recipient_key, channel_id) inbox.
-- The counter is atomically incremented on each enqueue via an UPSERT so
-- sequence numbers are gapless even under concurrent inserts.
--
-- Requires SQLite >= 3.35 (RETURNING clause support; available on Ubuntu 22.04+).

ALTER TABLE deliveries ADD COLUMN seq INTEGER NOT NULL DEFAULT 0;

CREATE TABLE IF NOT EXISTS delivery_seq_counters (
    recipient_key BLOB    NOT NULL,
    channel_id    BLOB    NOT NULL,
    next_seq      INTEGER NOT NULL DEFAULT 0,
    PRIMARY KEY (recipient_key, channel_id)
);

-- Index lets ORDER BY seq queries use an index scan instead of a sort.
CREATE INDEX IF NOT EXISTS idx_del_seq
    ON deliveries (recipient_key, channel_id, seq);