fix(engagement): two defects the Phase 11b live walk found in core

Both are invisible to a fixture and loud on a real database, which is why the
walk is the phase's acceptance rather than a formality.

1. registerEngagementSeeds validated `max_sends_per_hour` and then dropped it
   from the normalized rule. The column is NOT NULL, so every one of the 25
   module-seeded rules failed to insert at boot. The registry test asserted the
   REJECTION of a bad ceiling and never that a good one survives; it now asserts
   the normalized rule against `engagementRules.db.insert`'s own column list, so
   the next field added is covered the day it is added.

2. The cooldown claim runs inside the engine's per-channel loop and its key was
   (rule, user, subject). So the first channel of a rule claimed the cooldown and
   every later one was reported as cooled -- and `inapp` is ranked first
   deliberately, so a rule naming email + in-app delivered the inbox item and
   silently never the mail. Core's own `news.post` rule has that shape. Phase
   11b's decision 8 requires the letter and the inbox item to fire together.

   `channel` joins the PRIMARY KEY (the org lead's decision 12: a cooldown is per
   delivery, not per occasion). Migrated in place behind an information_schema
   guard, because MariaDB has no conditional form of a key change and replaying
   schema.sql would otherwise fail on every boot after the first.

1551 core tests green; both new tests verified by reverting each fix in turn.

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

server/src/modules/registries.js

@@ -874,6 +874,10 @@ function checkSeedRule(owner, entry, ownTemplateKeys, coreKeys) {
     name: r.name,
     audience: r.audience,
     audience_segment_id: null,
+    // Checked above and carried here: the column is NOT NULL, so a normalizer
+    // that validates the ceiling and then drops it fails every insert in the
+    // group at boot — loudly, but only on a real database.
+    max_sends_per_hour: r.max_sends_per_hour,
     channels: [...r.channels],
     template_keys: { ...keys },
     conditions: r.conditions === undefined ? null : r.conditions,