Boot: seed the identity legacy map with one INSERT … SELECT per source (the per-row loop over 28k guests was 6.4 s of every boot) (#269)

ddiff --git a/server/identity/subjects.js b/server/identity/subjects.js
iindex 3f04ae2b..a7c687d9 100644
---- a/server/identity/subjects.js
++++ b/server/identity/subjects.js
@@@ -47,23 +47,20 @@ async function backfill(db) {
   * reported through /internal/link-account.
   */
  async function seedLegacyMap(db) {
--    const ins = db.prepare(`INSERT INTO identity_legacy_map (source_system, source_type, source_id, subject_id, verified_at)
--                            VALUES (?, ?, ?, ?, ov_now()) ON CONFLICT DO NOTHING`);
++    // One INSERT … SELECT per source, not one INSERT per row: with 28k anonymous sessions the per-row loop was 6.4 s of
++    // every boot (2026-10-17), and a restart's queued requests wait for the boot.
++    const into = `INSERT INTO identity_legacy_map (source_system, source_type, source_id, subject_id, verified_at)`;
      let n = 0;
      await db.tx(async () => {
--        for (const u of await db.prepare('SELECT id, subject_id, legacy_source, legacy_id FROM users WHERE subject_id IS NOT NULL').all()) {
--            n += (await ins.run(SELF, 'user', String(u.id), u.subject_id)).changes;
--            if (u.legacy_source && u.legacy_id != null) n += (await ins.run(String(u.legacy_source), 'user', String(u.legacy_id), u.subject_id)).changes;
--        }
--        for (const g of await db.prepare('SELECT id, subject_id FROM anon_users WHERE subject_id IS NOT NULL').all()) {
--            n += (await ins.run(SELF, 'anon_user', String(g.id), g.subject_id)).changes;
--        }
++        const run = async (select) => { n += (await db.prepare(`${into} ${select} ON CONFLICT DO NOTHING`).run()).changes; };
++        await run(`SELECT '${SELF}', 'user', CAST(id AS TEXT), subject_id, ov_now() FROM users WHERE subject_id IS NOT NULL`);
++        await run(`SELECT CAST(legacy_source AS TEXT), 'user', CAST(legacy_id AS TEXT), subject_id, ov_now() FROM users
++                   WHERE subject_id IS NOT NULL AND legacy_source IS NOT NULL AND legacy_source <> '' AND legacy_id IS NOT NULL`);
++        await run(`SELECT '${SELF}', 'anon_user', CAST(id AS TEXT), subject_id, ov_now() FROM anon_users WHERE subject_id IS NOT NULL`);
          // Sites that report their own local id through /internal/link-account (Live does). Rows written by
          // the OAuth exchange hold 'network:<id>' instead and say nothing about the site's ids.
--        for (const l of await db.prepare(`SELECT la.service, la.service_user_id, u.subject_id FROM linked_accounts la JOIN users u ON u.id = la.user_id
--                                    WHERE u.subject_id IS NOT NULL AND la.service_user_id NOT LIKE 'network:%'`).all()) {
--            if (/^[a-z][a-z0-9-]{1,39}$/.test(l.service)) n += (await ins.run(l.service, 'user', String(l.service_user_id), l.subject_id)).changes;
--        }
++        await run(`SELECT la.service, 'user', CAST(la.service_user_id AS TEXT), u.subject_id, ov_now() FROM linked_accounts la JOIN users u ON u.id = la.user_id
++                   WHERE u.subject_id IS NOT NULL AND la.service_user_id NOT LIKE 'network:%' AND la.service ~ '^[a-z][a-z0-9-]{1,39}$'`);
      });
      return n;
  }
ddiff --git a/test/identity-subjects.test.js b/test/identity-subjects.test.js
iindex 1c1048ae..b40da8c4 100644
---- a/test/identity-subjects.test.js
++++ b/test/identity-subjects.test.js
@@@ -39,6 +39,11 @@ assert.strictEqual(alex.subject_id.slice(4, 14), contracts.ids.ulid(Date.parse('
  const before = alex.subject_id;
  console.log = () => {}; await subjects.ensureSchema(db); console.log = quiet;   // boot again: nothing changes
  assert.strictEqual((await db.prepare('SELECT subject_id FROM users WHERE id = 1').get()).subject_id, before, 'subject ids are stable across boots');
++// The set-based seed: network users 1 and 2, Live's legacy 77, guest 5 and Live's reported 321; the OAuth exchange's
++// 'network:1' says nothing about Tools' ids. A second boot adds nothing.
++const mapRows = await db.prepare('SELECT source_system, source_type, source_id FROM identity_legacy_map ORDER BY 1, 2, 3').all();
++assert.deepStrictEqual(mapRows.map((r) => `${r.source_system}:${r.source_type}:${r.source_id}`),
++    ['live:user:321', 'live:user:77', 'network:anon_user:5', 'network:user:1', 'network:user:2']);
  
  // Seeded map: network self ids and the Live legacy id.
  assert.strictEqual((await subjects.resolve(db, { source_system: 'network', source_type: 'user', source_id: 1 })).subject.id, before);