perf: partial index for the rejected outbox count (#253)

ddiff --git a/migrations/0014_outbox_rejected_index.sql b/migrations/0014_outbox_rejected_index.sql
nnew file mode 100644
iindex 0000000..032c710
---- /dev/null
++++ b/migrations/0014_outbox_rejected_index.sql
@@@ -0,0 +1,7 @@
++-- phase: expand
++-- no-transaction
++-- openvibe-sdk's outbox.status().rejected reads the whole outbox for its count
++-- (SELECT count(*) FROM event_outbox WHERE rejected_at IS NOT NULL): 625 ms and 79,944
++-- pages on 2026-11-08, with 0 rejected rows. A partial index holds only the rejected
++-- rows, so the same count is an index-only scan over an index that is usually empty.
++CREATE INDEX CONCURRENTLY IF NOT EXISTS event_outbox_rejected ON event_outbox (rejected_at) WHERE rejected_at IS NOT NULL;
ddiff --git a/test/outbox-rejected-index.test.js b/test/outbox-rejected-index.test.js
nnew file mode 100644
iindex 0000000..9ca61e6
---- /dev/null
++++ b/test/outbox-rejected-index.test.js
@@@ -0,0 +1,35 @@
++'use strict';
++/**
++ * The rejected-events count on /api/ready and the metrics gauges (openvibe-sdk outbox.status()) walks the
++ * whole outbox without an index: 625 ms and 79,944 shared pages on the 2026-11-08 production read, for a
++ * count that was zero. Migration 0014 adds a partial index over the rejected rows only; this proves the
++ * count can use it (an index-only scan) and still answers the same.
++ */
++const assert = require('assert');
++const { testDb } = require('./helpers/db');
++
++(async () => {
++    const { db } = await testDb();
++    try {
++        const indexes = await db.prepare(`SELECT indexname FROM pg_indexes
++            WHERE schemaname = current_schema() AND tablename = 'event_outbox'`).all();
++        assert.ok(indexes.some((r) => r.indexname === 'event_outbox_rejected'), 'migration 0014 created the partial index');
++
++        const now = Date.now();
++        await db.prepare(`INSERT INTO event_outbox (event_id, envelope, created_at, next_attempt_at) VALUES
++            ('evt_sent', '{}', ?, ?), ('evt_rejected', '{}', ?, ?)`).run(now, now, now, now);
++        await db.prepare('UPDATE event_outbox SET sent_at = ? WHERE event_id = ?').run(now, 'evt_sent');
++        await db.prepare('UPDATE event_outbox SET rejected_at = ? WHERE event_id = ?').run(now, 'evt_rejected');
++
++        const count = await db.prepare('SELECT count(*) AS n FROM event_outbox WHERE rejected_at IS NOT NULL').get();
++        assert.strictEqual(Number(count.n), 1, 'the count still sees the one rejected event');
++
++        await db.query('SET enable_seqscan = off');
++        const plan = (await db.prepare('EXPLAIN SELECT count(*) FROM event_outbox WHERE rejected_at IS NOT NULL').all())
++            .map((r) => r['QUERY PLAN']).join('\n');
++        assert.match(plan, /event_outbox_rejected/, plan);
++    } finally {
++        await db.close();
++    }
++    console.log('ok - the rejected outbox count is an index-only scan over event_outbox_rejected');
++})().catch((err) => { console.error(err); process.exit(1); });