docs: capture DNS resilience and Technitium backup sync

docs/servers/SERENITY.md

@@ -53,6 +53,15 @@ Text Embeddings Inference reranker for OpenWebUI RAG pipeline.
 
 Managed via Unraid Community Apps. See Unraid dashboard for individual service ports and status.
 
+### Technitium Backup Resolver
+
+Serenity hosts the secondary backup Technitium node on `10.5.30.10`.
+
+- Live stack path: `/mnt/user/appdata/technitium-serenity`
+- Resolver bind IP: `10.5.30.10`
+- Config is refreshed from PD's live Technitium directory by the PD-side sync runner every 15 minutes
+- Same-host checks from Serenity to `10.5.30.10` are unreliable because the resolver sits behind macvlan networking; verify from another LAN peer instead
+
 ## Notes
 
 - No GPU; reranker uses CPU-only TEI image