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/src/model/settings/settings.db.js

@@ -36,6 +36,24 @@ async function seedDefault(key, value) {
   await query('INSERT IGNORE INTO settings (`key`, value) VALUES (?, ?)', [key, value])
 }
 
+/**
+ * Take a one-shot guard, atomically. `true` means THIS caller wrote the row.
+ *
+ * The same `INSERT IGNORE` as `seedDefault`, and the difference is the whole
+ * point: this one reports whether it won. A guard read with `get()` and written
+ * later with `set()` is not a guard at all under concurrency — two processes
+ * both read "absent" and both proceed — and this is used where proceeding twice
+ * means seeding a rule group twice, i.e. two mails per event.
+ *
+ * The atomicity is the PRIMARY KEY's: exactly one INSERT can create a given
+ * `key`, so exactly one caller sees `affectedRows === 1`. No transaction and no
+ * lock, the same bargain `engagementWorker`'s claim makes.
+ */
+async function claim(key, value) {
+  const res = await query('INSERT IGNORE INTO settings (`key`, value) VALUES (?, ?)', [key, value])
+  return Number(res && res.affectedRows) === 1
+}
+
 // Delete a settings row. "Reset to defaults" for the theming/nav keys is the
 // *absence* of a row, not a stored copy of the defaults — see
 // docs/website/THEMING_AND_NAV.md §2. Deleting a key that was never set is a
@@ -44,4 +62,4 @@ async function remove(key) {
   await query('DELETE FROM settings WHERE `key` = ?', [key])
 }
 
-module.exports = { getAll, get, getRow, set, seedDefault, remove }
+module.exports = { getAll, get, getRow, set, seedDefault, claim, remove }