README: replace Mermaid diagram with real drawio SVG export

No more Mermaid, per standing rule - the architecture diagram is now
a real image (assets/architecture-overview.svg), built and exported
from the Nextcloud Diagramming app, fetched via WebDAV and validated
as well-formed XML before committing. Ends the rendering-gap chase
entirely since there's no live renderer involved anymore, just a
static image.

README.md

@@ -76,46 +76,7 @@ This repo is a sanitized showcase of the systems behind that platform — the ac
 
 ## Architecture at a glance
 
-```mermaid
-flowchart LR
-    Internet(["Internet"]) --> Edge["Edge Firewall"]
-    Edge --> LAN["LAN"]
-
-    LAN --> DNS["Internal DNS<br/>two zones"]
-    LAN --> Vault["Vault<br/>PKI + secrets"]
-    DNS --> Vault
-
-    Vault --> Apps["App Host<br/>41 stacks"]
-    Vault --> Automation["Automation VM<br/>Git + orchestration"]
-    LAN --> FileServer["File Server"]
-    DNS --> FileServer
-    LAN --> Automation
-
-    Apps --> InternalProxy["Internal Proxy Zone"]
-    Apps --> SSO["SSO"]
-
-    Vault --> EdgeProxy["User-Facing Proxy<br/>mTLS"]
-    DNS --> EdgeProxy
-    Apps -.proxied.-> EdgeProxy
-    SSO -.auth.-> EdgeProxy
-
-    Apps -.-> Monitoring["Monitoring"]
-    Automation -.-> Monitoring
-    FileServer -.-> Monitoring
-    EdgeProxy -.-> Monitoring
-
-    Apps -.-> SecMon["Security Monitoring"]
-    Automation -.-> SecMon
-    FileServer -.-> SecMon
-    EdgeProxy -.-> SecMon
-
-    classDef edge fill:#f8cecc,stroke:#b85450,color:#333
-    classDef core fill:#dae8fc,stroke:#6c8ebf,color:#333
-    classDef obs fill:#fff2cc,stroke:#d6b656,color:#333
-    class Internet,Edge edge
-    class LAN,DNS,Vault,Apps,Automation,FileServer,InternalProxy,SSO,EdgeProxy core
-    class Monitoring,SecMon obs
-```
+![Architecture Overview](assets/architecture-overview.svg)
 Solid arrows are real dependencies; dotted arrows are asynchronous telemetry reporting, not a blocking dependency. Full breakdown of all 8 traffic laws (ingress, east-west, secrets, DNS, observability, notifications, CI/CD, management) with the actual trust-boundary reasoning: [docs/architecture-overview.md](docs/architecture-overview.md).
 
 ## Systems