fix(security): SSRF protection for alarm webhooks (FLAW-05)

Discord and custom webhook URLs are checked when saved and before every
call: https only, Discord URLs must point to discord.com. A guarded HTTP
client resolves hosts itself and connects only to allowed IPs; localhost,
link-local and cloud metadata are always blocked, LAN ranges (incl.
Tailscale CGNAT) and plain http only with the new admin switch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/settings/AdminSettingsView.tsx

@@ -132,6 +132,7 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
           smtpFrom: sys.smtpFrom || '',
           smtpTls: sys.smtpTls ?? true,
           smtpSkipVerify: Boolean(sys.smtpSkipVerify),
+          allowPrivateWebhooks: Boolean(sys.allowPrivateWebhooks),
           smtpConfiguredViaEnv: Boolean(sys.smtpConfiguredViaEnv),
         });
       }
@@ -1081,6 +1082,22 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
                   )}
                 </div>
 
+                {/* Alarm webhooks into the local network (SSRF protection) */}
+                <div className="p-4 bg-canvas border border-border rounded-card flex items-center justify-between gap-4">
+                  <div className="pr-4">
+                    <p className="font-bold text-main">Alarm-Webhooks ins lokale Netz erlauben</p>
+                    <p className="text-[11px] text-muted mt-0.5">
+                      Standard: Discord- und Webhook-Alarme gehen nur an öffentliche https-Adressen. Eingeschaltet sind auch Ziele
+                      im Heimnetz erlaubt (z. B. Home Assistant unter 192.168.x.x oder über Tailscale), dort auch per http.
+                      Der Server selbst (localhost) und Cloud-Metadaten bleiben immer gesperrt.
+                    </p>
+                  </div>
+                  <ToggleSwitch
+                    checked={Boolean(settings.allowPrivateWebhooks)}
+                    onChange={(allowPrivateWebhooks) => handleSaveSystemSettings({ allowPrivateWebhooks })}
+                  />
+                </div>
+
                 {/* Public Registration Toggle Switch */}
                 <div className="p-4 bg-canvas border border-border rounded-card flex items-center justify-between gap-4">
                   <div className="pr-4">

frontend/src/lib/types.ts

@@ -178,6 +178,7 @@ export interface SystemSettings {
   smtpFrom?: string;
   smtpTls?: boolean;
   smtpSkipVerify?: boolean;
+  allowPrivateWebhooks?: boolean; // alarm webhooks may reach the LAN (and plain http)
   smtpConfiguredViaEnv?: boolean;
 }