refactor(18): IN-01/IN-02 reuse fetched row for GET timezone fallback

The GET /config/timezone handler SELECTed app_config then, on the unset
path, called getHouseholdTimezone(db) which re-issued the identical
SELECT before falling back (IN-01). The fallback decision also lived in
two places (IN-02). Route the handler through the centralized
resolveHouseholdTimezone(row?.value) added for WR-01: no redundant
round-trip, single source for the D-06 policy. Behavior unchanged.

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

apps/api/src/routes/admin.ts

@@ -27,7 +27,7 @@ import { eq, sql } from 'drizzle-orm';
 import { db } from '../db/client.js';
 import { users, memberCredentials, calendars, appConfig } from '../db/schema.js';
 import { requireAdmin } from '../lib/requireAdmin.js';
-import { isValidIanaTimezone, getHouseholdTimezone } from '../lib/householdTimezone.js';
+import { isValidIanaTimezone, resolveHouseholdTimezone } from '../lib/householdTimezone.js';
 import {
   validateEncryptAndStoreCredential,
   CredentialValidationError,
@@ -205,9 +205,9 @@ adminRouter.get('/config/timezone', async (c) => {
     .limit(1);
 
   const isExplicitlySet = row?.value != null;
-  const timezone = isExplicitlySet
-    ? (row.value as string)
-    : await getHouseholdTimezone(db);
+  // IN-01/IN-02: reuse the row we just SELECTed and let the centralized accessor apply
+  // the D-06 fallback — no second app_config round-trip, single source for the policy.
+  const timezone = resolveHouseholdTimezone(row?.value ?? null);
 
   return c.json({ timezone, isExplicitlySet });
 });