feat: add affiliate invite rebate flow and admin rebate-rate setting

frontend/src/views/admin/SettingsView.vue

@@ -2153,6 +2153,31 @@
                     {{ t("admin.settings.defaults.defaultBalanceHint") }}
                   </p>
                 </div>
+                <div>
+                  <label
+                    class="mb-2 block text-sm font-medium text-gray-700 dark:text-gray-300"
+                  >
+                    {{ t("admin.settings.defaults.affiliateRebateRate") }}
+                  </label>
+                  <div class="relative">
+                    <input
+                      v-model.number="form.affiliate_rebate_rate"
+                      type="number"
+                      step="0.01"
+                      min="0"
+                      max="100"
+                      class="input pr-8"
+                      placeholder="20"
+                    />
+                    <span
+                      class="pointer-events-none absolute right-3 top-1/2 -translate-y-1/2 text-gray-400"
+                      >%</span
+                    >
+                  </div>
+                  <p class="mt-1.5 text-xs text-gray-500 dark:text-gray-400">
+                    {{ t("admin.settings.defaults.affiliateRebateRateHint") }}
+                  </p>
+                </div>
                 <div>
                   <label
                     class="mb-2 block text-sm font-medium text-gray-700 dark:text-gray-300"
@@ -4972,6 +4997,7 @@ const form = reactive<SettingsForm>({
   totp_enabled: false,
   totp_encryption_key_configured: false,
   default_balance: 0,
+  affiliate_rebate_rate: 20,
   default_concurrency: 1,
   default_subscriptions: [],
   force_email_on_third_party_signup: false,
@@ -5894,6 +5920,10 @@ async function saveSettings() {
       password_reset_enabled: form.password_reset_enabled,
       totp_enabled: form.totp_enabled,
       default_balance: form.default_balance,
+      affiliate_rebate_rate: Math.min(
+        100,
+        Math.max(0, Number(form.affiliate_rebate_rate) || 0),
+      ),
       default_concurrency: form.default_concurrency,
       default_subscriptions: normalizedDefaultSubscriptions,
       force_email_on_third_party_signup: form.force_email_on_third_party_signup,