fix(engagement): claim the seed guard atomically, and let a rule name a digest body

Both defects came out of Phase 13's acceptance walk, and neither was visible to
any test.

**1. The one-shot rule-group guard was not a guard.** `seedRuleGroup` and
`coreRules.seedGroup` both read their settings stamp, inserted the whole group,
and wrote the stamp AFTER the loop. Two instances booting in the same moment both
read "absent" and both insert -- the walk ended up with 52 module rules where the
module ships 26. `docker compose up --scale app=2` and a rolling restart both
start two instances on purpose, so this is ordinary rather than exotic.

`settings.db` gains `claim(key, value)`: the same `INSERT IGNORE` as
`seedDefault`, reporting its own `affectedRows`, so exactly one caller can win a
key. The atomicity is the PRIMARY KEY's -- no transaction, no lock, the same
bargain `engagementWorker`'s row claim already makes. Both seeders claim before
inserting.

The trade the code already documented is unchanged, only its order: a process
that dies mid-loop leaves the group stamped and partly seeded, and the missing
rules are an operator's visit to the "new rule" form. A duplicate rule is two
mails per event, for every rule in the group, which both functions' own comments
already call the worse outcome.

**Why no test caught it:** the stubs supplied `get` and `set` over a Map, which
cannot race, so they agreed with the bug -- Phase 4a's `foundRows` finding in a
different costume. The fake is now `claim`-shaped and decides without yielding,
exactly as the table does, and both suites gained a test that runs two seeders
with `Promise.all` and asserts one insert each. Verified by reverting the fix:
the module test then reports every rule inserted twice.

**2. A rule that names a `digest` body could not be saved.** `registries.js`
`checkSeedRule` permits `digest` in as many words -- it is the body
`teamDigestWorker` renders for a rule whose email channel an individual set to
digest mode, so it never appears in `channels` and never could -- and
MODULE_API.md 2.4 tells modules they may point `template_keys` at
`notify.digest`. `engagementRules.model.js` then rejected any key that was not
one of the rule's channels.

So every rule shipping a digest body answered an operator who opened it and
pressed Save with a 400 naming a key they had never typed: core's own Team and
news rules, and sixteen of module-uo's. The only way to save was to delete the
digest body, silently dropping digest support from that rule. The two validators
now agree; anything that is neither a channel nor `digest` is still refused, with
a test for each direction.

No MODULE_API bump: no member is added, removed or changed, and the documented
contract is what the code now honours rather than something new.

Co-Authored-By: Claude <noreply@anthropic.com>

server/test/engagementEngine.test.js

@@ -627,6 +627,44 @@ test('the hourly ceiling counts sends, not attempts', async () => {
   assert.equal(result.enqueued, 1)
 })
 
+// ── templateKeys: what a channel is, and what `digest` is ──────────────────
+
+test('a rule may name a `digest` body, which is a template slot rather than a channel', async () => {
+  // The defect Phase 13's acceptance walk found. `registries.js` `checkSeedRule`
+  // permits `digest` in as many words — it is the body `teamDigestWorker`
+  // renders for a rule whose email channel an individual set to digest mode, so
+  // it never appears in `channels` and never could — and core's own Team and
+  // news rules ship one, as do sixteen of module-uo's. This validator rejected
+  // it, so every one of those rules answered an operator who opened it and
+  // pressed Save with a 400 naming a key they had never typed, and the only way
+  // to save was to delete the digest body.
+  const checked = await rules.validate({
+    triggerId: 'uo.house.idoc_warning',
+    name: 'IDOC warning',
+    channels: ['email'],
+    audience: 'owner',
+    templateKeys: { email: 'notify.event', digest: 'notify.digest' },
+  })
+
+  assert.equal(checked.ok, true, checked.errors && checked.errors.join(' '))
+  assert.equal(checked.rule.template_keys.digest, 'notify.digest')
+})
+
+test('a templateKeys entry that is neither a channel nor `digest` is still refused', async () => {
+  // The rule that was right all along, kept: `digest` is one named exception
+  // with a renderer behind it, not a hole that admits any word.
+  const checked = await rules.validate({
+    triggerId: 'uo.house.idoc_warning',
+    name: 'IDOC warning',
+    channels: ['email'],
+    audience: 'owner',
+    templateKeys: { email: 'notify.event', carrierpigeon: 'notify.event' },
+  })
+
+  assert.equal(checked.ok, false)
+  assert.match(checked.errors.join(' '), /carrierpigeon/)
+})
+
 // ── Ceilings: the security boundary, both halves ───────────────────────────
 
 test('a rule may not be SAVED with an audience wider than its trigger permits', async () => {