fix(i18n): deep-merge en.json as fallback for locales missing keys

D12 of master-plan-21 assumed next-intl had a built-in fallback to EN
already configured. It did not — request.ts loaded only
messages/${locale}.json and no getMessageFallback was defined, so any
key absent in the user's locale rendered the key path literally
(for example "memory.concept.title").

Plan 21 added 156 memory.* keys to en and pt-BR but the other 39
locales kept only the pre-existing 36 memory.* keys, so users on those
locales saw raw key paths across the new Memory studio.

Fix: load en.json as the base and deep-merge the locale-specific
messages on top. Existing translations are untouched; only missing
keys fall back to English. Satisfies §7 "i18n 41 locales".

src/i18n/request.ts

@@ -3,6 +3,34 @@ import { cookies, headers } from "next/headers";
 import { LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE } from "./config";
 import type { Locale } from "./config";
 
+// Deep-merge fallback messages so locales that lack newer keys (e.g. memory.*
+// added by plan 21) silently fall back to EN instead of rendering the key path.
+function mergeMessages(
+  fallback: Record<string, unknown>,
+  override: Record<string, unknown>
+): Record<string, unknown> {
+  const out: Record<string, unknown> = { ...fallback };
+  for (const [k, v] of Object.entries(override)) {
+    const baseVal = out[k];
+    if (
+      v !== null &&
+      typeof v === "object" &&
+      !Array.isArray(v) &&
+      baseVal !== null &&
+      typeof baseVal === "object" &&
+      !Array.isArray(baseVal)
+    ) {
+      out[k] = mergeMessages(
+        baseVal as Record<string, unknown>,
+        v as Record<string, unknown>
+      );
+    } else {
+      out[k] = v;
+    }
+  }
+  return out;
+}
+
 export default getRequestConfig(async () => {
   // 1. Try cookie
   const cookieStore = await cookies();
@@ -19,10 +47,16 @@ export default getRequestConfig(async () => {
     locale = DEFAULT_LOCALE;
   }
 
-  const messages = (await import(`./messages/${locale}.json`)).default;
+  const enMessages = (await import("./messages/en.json"))
+    .default as Record<string, unknown>;
+  if (locale === "en") {
+    return { locale, messages: enMessages };
+  }
+  const localeMessages = (await import(`./messages/${locale}.json`))
+    .default as Record<string, unknown>;
 
   return {
     locale,
-    messages,
+    messages: mergeMessages(enMessages, localeMessages),
   };
 });