projects: cover art on every case study

Six generated covers wired in via an optional cover key - 2528x1696 sources
optimised to 1600px webp, 49-144KB each. They sit on the page background
rather than the diagrams' light card, since the art is already generated in
the site palette on near-black. Cover carries the story at a glance, the
diagram still carries the architecture below it.

src/content/projects/global-infra-modernisation.md

@@ -7,6 +7,7 @@ period: "2019 – 2025"
 stack: ["VMware / vSphere", "Azure (Blob, AVS)", "Microsoft 365", "SD-WAN", "Aruba ClearPass", "Palo Alto / FortiGate", "Veeam"]
 featured: false
 order: 40
+cover: "global-infra-modernisation"
 ---
 
 ## Problem