fix(whatsapp): admin-pinned template language + Arabic locale support (#647)

Reporter @Rekoo-PS hit three independent gaps trying to deliver an
Arabic Meta template. Bundled here because they fan out from the same
root cause (no first-class language config on the WhatsApp tab) and the
review surfaces are tightly coupled.

**1. Test send hardcoded `en_US` (`adminWhatsapp.js:141`).** Smoking gun
for "I can't make it work" — Meta returned template_not_found_in_language
(132001) on every test send for non-English templates, no matter what
else the admin configured. Replaced with `config.template_language ||
'en_US'`.

**2. No `template_language` field on `whatsapp_configs`.** The only
priors were per-message `data.language` (always null from our callers in
`adminEvents.js:854,1188`) and `app_settings.general_default_language`
(the *system UI* language, not the *template's* language registered with
Meta). Migration 137 adds the column; GET + PUT surface it; the
processor uses it as the highest-priority default when message_data
doesn't override.

Resolution order in `whatsappProcessor.processWhatsAppQueue` is now:
  1. message_data.language (per-event override — caller path TBD)
  2. config.template_language (admin-pinned template language)
  3. app_settings.general_default_language (system fallback)
  4. en_US (hardcoded last resort)

**3. `LANGUAGE_MAP` + `PASSWORD_LABELS` didn't cover Arabic.** Added
`ar` (Meta's single-code form per RFC; no region variant). For any
language we don't enumerate (e.g. Turkish `tr_TR`, Chinese `zh_CN`,
Hebrew `he_IL`), `resolveLanguageCode` now pass-throughs valid-shape
codes (lowercase-language + optional underscore + uppercase-region) and
forwards them to Meta as-is. If they don't match a registered template
Meta returns 132001, which the test route already surfaces back to the
admin via `error.message` — fail-loud, no silent fallback.

Validation:
- Unit smoke on `resolveLanguageCode` across 18 representative inputs
  (in-map, pass-through, canonicalization, rejection) — all behaviours
  correct.
- Lint clean on all 7 changed files.
- Frontend `tsc --noEmit` clean.
- Migration `node -c` syntax-checked; additive + `hasColumn`-guarded so
  re-running is safe.

Frontend: free-text input on the WhatsApp tab with EN + DE i18n.
Pointing at Meta's supported-languages docs via the hint text — Meta's
list grows; a hardcoded dropdown would rot.

Closes #647.

frontend/src/features/settings/tabs/WhatsAppTab.tsx

@@ -31,6 +31,7 @@ export const WhatsAppTab: React.FC = () => {
   const [wabaId, setWabaId] = useState('');
   const [accessToken, setAccessToken] = useState('');
   const [templateName, setTemplateName] = useState('gallery_ready');
+  const [templateLanguage, setTemplateLanguage] = useState('');
   const [enabled, setEnabled] = useState(false);
   const [showToken, setShowToken] = useState(false);
   const [testPhone, setTestPhone] = useState('');
@@ -43,6 +44,7 @@ export const WhatsAppTab: React.FC = () => {
       // Leave it visible-as-masked so the admin sees that a token exists.
       setAccessToken(data.access_token || '');
       setTemplateName(data.template_name || 'gallery_ready');
+      setTemplateLanguage(data.template_language || '');
       setEnabled(Boolean(data.enabled));
     }
   }, [data]);
@@ -53,6 +55,7 @@ export const WhatsAppTab: React.FC = () => {
       waba_id: wabaId,
       access_token: accessToken,
       template_name: templateName,
+      template_language: templateLanguage,
       enabled,
     }),
     onSuccess: () => {
@@ -175,6 +178,23 @@ export const WhatsAppTab: React.FC = () => {
             </p>
           </div>
 
+          <div>
+            <label className="block text-sm font-medium text-neutral-700 dark:text-neutral-300 mb-1">
+              {t('settings.whatsapp.templateLanguage', 'Template language')}
+            </label>
+            <Input
+              value={templateLanguage}
+              onChange={(e) => setTemplateLanguage(e.target.value)}
+              placeholder={t('settings.whatsapp.templateLanguagePlaceholder', 'e.g. en_US, de_DE, ar, pt_BR') as string}
+            />
+            <p className="mt-1 text-xs text-neutral-500 dark:text-neutral-400">
+              {t(
+                'settings.whatsapp.templateLanguageHint',
+                'Meta template language code, exactly as you registered it in Meta Business Manager (`ar`, `en_US`, `de_DE`, `pt_BR`, etc.). Leave empty to fall back to the system default language. Meta returns "template not found in language" if this doesn\'t match a registered template.',
+              )}
+            </p>
+          </div>
+
           <label className="flex items-center gap-2 text-sm text-neutral-800 dark:text-neutral-200">
             <input
               type="checkbox"