feat(security): harden gateway boundaries and usage policies

Consolidate subscription usage policy enforcement, privacy-safe persistence, and gateway security hardening into one reviewable change.

Includes bounded HTTP and execution envelopes, header and protocol guards, DNS and relay validation, authentication and secret projection hardening, secure backup/install paths, and regression coverage.

frontend/src/features/providers/components/ProviderMobileCard.vue

@@ -9,12 +9,12 @@
         <div class="flex items-center gap-1.5">
           <span class="font-medium text-foreground truncate">{{ provider.name }}</span>
           <a
-            v-if="provider.website"
-            :href="provider.website"
+            v-if="safeProviderWebsite"
+            :href="safeProviderWebsite"
             target="_blank"
             rel="noopener noreferrer"
             class="text-muted-foreground hover:text-primary transition-colors shrink-0"
-            :title="provider.website"
+            :title="safeProviderWebsite"
             @click.stop
           >
             <ExternalLink class="w-3.5 h-3.5" />
@@ -221,7 +221,7 @@
 </template>
 
 <script setup lang="ts">
-import { ref, watch } from 'vue'
+import { computed, ref, watch } from 'vue'
 import {
   Edit,
   Eye,
@@ -241,6 +241,7 @@ import { formatBillingType } from '@/utils/format'
 import { sortEndpoints, isEndpointAvailable, getEndpointDotColor, getEndpointTooltip } from '@/features/providers/composables/useEndpointStatus'
 import { isKeyManagedProviderType } from '../utils/providerTypeUtils'
 import { useI18n } from '@/i18n'
+import { safeExternalWebUrl } from '@/utils/navigationSecurity'
 
 const props = defineProps<{
   provider: ProviderWithEndpointsSummary
@@ -272,6 +273,7 @@ const vAutoFocus = {
 
 const localDescriptionValue = ref('')
 const { legacyT, locale } = useI18n()
+const safeProviderWebsite = computed(() => safeExternalWebUrl(props.provider.website))
 
 watch(
   () => props.editingDescriptionId,