fix: add SSRF protection for link preview and Immich URL

- Create server/src/utils/ssrfGuard.ts with checkSsrf() and createPinnedAgent()
  - Resolves DNS before allowing outbound requests to catch hostnames that
    map to private IPs (closes the TOCTOU gap in the old inline checks)
  - Always blocks loopback (127.x, ::1) and link-local/metadata (169.254.x)
  - RFC-1918, CGNAT (100.64/10), and IPv6 ULA ranges blocked by default;
    opt-in via ALLOW_INTERNAL_NETWORK=true for self-hosters running Immich
    on a local network
  - createPinnedAgent() pins node-fetch to the validated IP, preventing
    DNS rebinding between the check and the actual connection

- Replace isValidImmichUrl() (hostname-string check, no DNS resolution)
  with checkSsrf(); make PUT /integrations/immich/settings async
  - Audit log entry (immich.private_ip_configured) written when a user
    saves an Immich URL that resolves to a private IP
  - Response includes a warning field surfaced as a toast in the UI

- Replace ~20 lines of duplicated inline SSRF logic in the link-preview
  handler with a single checkSsrf() call + pinned agent

- Document ALLOW_INTERNAL_NETWORK in README, docker-compose.yml,
  server/.env.example, chart/values.yaml, chart/templates/configmap.yaml,
  and chart/README.md

client/src/pages/SettingsPage.tsx

@@ -145,7 +145,8 @@ export default function SettingsPage(): React.ReactElement {
   const handleSaveImmich = async () => {
     setSaving(s => ({ ...s, immich: true }))
     try {
-      await apiClient.put('/integrations/immich/settings', { immich_url: immichUrl, immich_api_key: immichApiKey || undefined })
+      const saveRes = await apiClient.put('/integrations/immich/settings', { immich_url: immichUrl, immich_api_key: immichApiKey || undefined })
+      if (saveRes.data.warning) toast.warn(saveRes.data.warning)
       toast.success(t('memories.saved'))
       // Test connection
       const res = await apiClient.get('/integrations/immich/status')