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.config.ts

@@ -18,6 +18,9 @@ const projects = defineCollection({
     order: z.number().default(50),
     // Optional diagram key → rendered by the Diagram component (M3).
     diagram: z.string().optional(),
+    // Optional cover art → /covers/<slug>.webp, rendered above the case header.
+    // Sells the story at a glance; the diagram still carries the architecture.
+    cover: z.string().optional(),
     links: z
       .array(z.object({ label: z.string(), href: z.string().url() }))
       .optional(),