refactor(18): IN-03 memoize household timezone per outbox drain cycle

The UPDATE and CREATE all-day branches each called getHouseholdTimezone(db)
independently, so a drain processing both an all-day create row and an
all-day update row issued two identical app_config SELECTs. Add a lazy
per-cycle TimezoneResolver (mirroring the existing clientCache thread-through)
created in runOutboxDrain and passed into dispatchRow. The read stays lazy —
cycles with no all-day work never touch the DB — but is shared across all
all-day rows in a cycle. Behavior unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/src/broker/outboxWorker.ts

@@ -373,7 +373,31 @@ interface DispatchResult {
   error?: string;
 }
 
-async function dispatchRow(row: OutboxRow): Promise<DispatchResult> {
+/**
+ * Lazily resolves the household timezone at most once, caching the promise.
+ * Threaded through a drain cycle (mirroring clientCache, IN-01) so an all-day
+ * create row and an all-day update row in the same cycle share a single
+ * app_config read instead of issuing two identical SELECTs (IN-03). The read is
+ * still lazy: cycles with no all-day work never touch the DB.
+ */
+type TimezoneResolver = () => Promise<string>;
+
+function makeTimezoneResolver(): TimezoneResolver {
+  let cached: Promise<string> | undefined;
+  return () => {
+    if (cached === undefined) {
+      // D-05: route through the single stored-TZ accessor (no inline fallback duplicated here).
+      // D-06: getHouseholdTimezone falls back to process.env.TZ → Intl when no row is stored.
+      cached = getHouseholdTimezone(db);
+    }
+    return cached;
+  };
+}
+
+async function dispatchRow(
+  row: OutboxRow,
+  resolveTimezone: TimezoneResolver,
+): Promise<DispatchResult> {
   // CR-03: fail closed on credential errors — let loadClientForUser throw.
   // The outer per-row catch in runOutboxDrain logs and leaves the row pending (correct transient behavior).
   // Do NOT add an empty-credential fallback — that would silently PUT with no authentication.
@@ -499,9 +523,8 @@ async function dispatchRow(row: OutboxRow): Promise<DispatchResult> {
       fields.allDay &&
       fields.start
     ) {
-      // D-05: route through the single stored-TZ accessor (no inline fallback duplicated here).
-      // D-06: getHouseholdTimezone falls back to process.env.TZ → Intl when no row is stored.
-      const tz = await getHouseholdTimezone(db);
+      // IN-03: shared per-cycle resolver — one app_config read across all-day rows.
+      const tz = await resolveTimezone();
       const leadDays = fields.reminderLeadMinutes / 1440;
       allDayAlertInstantUtcUpdate = computeAlertInstantUtc(fields.start, leadDays, tz);
     }
@@ -607,9 +630,8 @@ async function dispatchRow(row: OutboxRow): Promise<DispatchResult> {
     // carries an explicit picker value or no reminder at all; no preserve path needed).
     let allDayAlertInstantUtcCreate: Date | undefined;
     if (fields.reminderLeadMinutes != null && fields.allDay && fields.start) {
-      // D-05: route through the single stored-TZ accessor (no inline fallback duplicated here).
-      // D-06: getHouseholdTimezone falls back to process.env.TZ → Intl when no row is stored.
-      const tz = await getHouseholdTimezone(db);
+      // IN-03: shared per-cycle resolver — one app_config read across all-day rows.
+      const tz = await resolveTimezone();
       const leadDays = fields.reminderLeadMinutes / 1440;
       allDayAlertInstantUtcCreate = computeAlertInstantUtc(fields.start, leadDays, tz);
     }
@@ -747,6 +769,10 @@ export async function runOutboxDrain(): Promise<void> {
     // credential at most once per cycle. Discarded when the drain returns — never persisted.
     const clientCache = new Map<number, FastmailClient>();
 
+    // IN-03: per-drain-cycle timezone resolver so multiple all-day rows in the same cycle
+    // share one app_config read. Lazy: cycles with no all-day work never hit the DB.
+    const resolveTimezone = makeTimezoneResolver();
+
     for (const row of sorted) {
       // D-04 fast path: if the create for this group already failed in this batch, skip the delete
       if (row.operation === 'delete' && row.groupId && failedCreateGroups.has(row.groupId)) {
@@ -793,7 +819,7 @@ export async function runOutboxDrain(): Promise<void> {
       }
 
       try {
-        const result = await dispatchRow(row);
+        const result = await dispatchRow(row, resolveTimezone);
 
         if (result.conflict) {
           // WR-06: distinguish an edit-as-move create-412 from a same-calendar conflict.