Implement TURN server (coturn) for WebRTC video calls

Add coturn Deployment with hostNetwork mode and init container for secret substitution. Include SOPS-encrypted shared secret, TLS certificate for turn.axion1337.chat, and Synapse TURN configuration with proper relay URIs and credentials.

Resolves DTLS timeout issues in RTC video calls by providing media relay for clients behind NAT/Firewall.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

apps/production/kustomization.yaml

@@ -3,12 +3,16 @@ kind: Kustomization
 resources:
   - matrix-postgres-auth.yaml
   - cert-issuer.yaml
+  - matrix-certificates.yaml
   # Neue Dateien:
   - custom-configs/synapse-values.yaml
   - custom-configs/element-values.yaml
   - custom-configs/mas-secret.yaml
   - element-web-docs-configmap.yaml
   - element-web-docs-server.yaml
+  # TURN Server für WebRTC
+  - coturn-secret.yaml
+  - coturn.yaml
   # HelmRelease (muss ganz unten stehen, damit die ConfigMaps vorher da sind!)
   - element-server-suite.yaml
   # Custom Apex Ingress für Element Web + Well-Known auf axion1337.chat