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/model/engagement/engagementCooldowns.db.js

@@ -1,8 +1,14 @@
 const { query } = require('../../utils/db')
 
 /**
- * Claim a fire for (rule, user, subject), or refuse it because the pair is still
- * cooling. ENGAGEMENT.md §4.1.
+ * Claim a fire for (rule, user, subject, channel), or refuse it because that
+ * delivery is still cooling. ENGAGEMENT.md §4.1.
+ *
+ * **`channel` is part of the key, and Phase 11b is where that was settled.** The
+ * engine claims inside its per-channel loop, so a key without the channel means
+ * the first channel of a two-channel rule claims the cooldown and every later one
+ * is refused as cooling - which made every in-universe email body of Phase 11b
+ * unreachable behind the in-app one. A cooldown is per delivery.
  *
  * **Two statements, each of which is its own atomic decision** - and it is worth
  * saying why it is not the single `INSERT ... ON DUPLICATE KEY UPDATE` §4.1
@@ -37,28 +43,29 @@ const { query } = require('../../utils/db')
  * `cooldown_seconds = 0` always passes, which is the documented meaning of a rule
  * with no cooldown: the guard becomes `last_fired_at <= now`, and it is.
  */
-async function claim(ruleId, userId, subjectKey, cooldownSeconds, now = new Date()) {
+async function claim(ruleId, userId, subjectKey, channel, cooldownSeconds, now = new Date()) {
   const moved = await query(
     `UPDATE engagement_cooldowns
         SET last_fired_at = ?, fire_count = fire_count + 1
-      WHERE rule_id = ? AND user_id = ? AND subject_key = ?
+      WHERE rule_id = ? AND user_id = ? AND subject_key = ? AND channel = ?
         AND last_fired_at <= ? - INTERVAL ? SECOND`,
-    [now, ruleId, userId, subjectKey, now, cooldownSeconds],
+    [now, ruleId, userId, subjectKey, channel, now, cooldownSeconds],
   )
   if (Number(moved?.affectedRows || 0) === 1) return true
 
   const inserted = await query(
-    `INSERT IGNORE INTO engagement_cooldowns (rule_id, user_id, subject_key, last_fired_at, fire_count)
-     VALUES (?, ?, ?, ?, 1)`,
-    [ruleId, userId, subjectKey, now],
+    `INSERT IGNORE INTO engagement_cooldowns (rule_id, user_id, subject_key, channel, last_fired_at, fire_count)
+     VALUES (?, ?, ?, ?, ?, 1)`,
+    [ruleId, userId, subjectKey, channel, now],
   )
   return Number(inserted?.affectedRows || 0) === 1
 }
 
-const get = async (ruleId, userId, subjectKey) => {
+const get = async (ruleId, userId, subjectKey, channel) => {
   const [row] = await query(
-    'SELECT * FROM engagement_cooldowns WHERE rule_id = ? AND user_id = ? AND subject_key = ?',
-    [ruleId, userId, subjectKey],
+    `SELECT * FROM engagement_cooldowns
+      WHERE rule_id = ? AND user_id = ? AND subject_key = ? AND channel = ?`,
+    [ruleId, userId, subjectKey, channel],
   )
   return row || null
 }