feat: websearch quota enhancements and balance notify hint

- QuotaLimit changed to *int64 (null=unlimited, >0=limited)
- Add reset-usage endpoint (POST /admin/settings/web-search-emulation/reset-usage)
- Show quota usage in header always (collapsed and expanded)
- Add reset quota button in expanded provider view
- Quota input: empty=unlimited with ∞ placeholder, must be >0 if set
- Add email verification hint on balance notify card

frontend/src/components/user/profile/ProfileBalanceNotifyCard.vue

@@ -48,6 +48,7 @@
         <!-- Email list with toggles -->
         <div>
           <label class="input-label">{{ t('profile.balanceNotify.extraEmails') }}</label>
+          <p class="mb-2 text-xs text-yellow-600 dark:text-yellow-400">{{ t('profile.balanceNotify.extraEmailsHint') }}</p>
 
           <!-- Saved email entries -->
           <div v-if="emailEntries.length > 0" class="space-y-2 mb-3">