fix: prevent watcher OOM by tuning eleventy-img plugin

- transformOnRequest: process images on-demand in watch mode instead
  of all at once during rebuild (same pattern as zachleat.com)
- cacheOptions: cache remote image fetches to disk (1d build, 30d watch)
- concurrency: 4 (down from default ~10 based on CPU count) to limit
  Sharp's native memory usage from parallel image decodes

Root cause: Sharp processes remote images outside V8 heap, so
--max-old-space-size doesn't cap total memory. Large remote images
(e.g. 3072px-wide) at concurrency 10 spike native memory enough
to exceed the 3GB cgroup limit.

Confab-Link: http://localhost:8080/sessions/0ec83454-d346-4329-8aaf-6b12139bf596

eleventy.config.js

@@ -229,11 +229,21 @@ export default function (eleventyConfig) {
   });
 
   // Image optimization - transforms <img> tags automatically
+  // transformOnRequest: in watch/serve mode, process images on-demand instead of all
+  // at once during rebuild — prevents Sharp from saturating memory with parallel decodes.
+  // cacheOptions: cache remote image fetches to disk so restarts don't re-download everything.
+  // concurrency: limit parallel Sharp operations (default scales to CPU count, ~10 on this
+  // server). Each large remote image can consume 20-50MB of native memory outside V8 heap.
   eleventyConfig.addPlugin(eleventyImageTransformPlugin, {
     extensions: "html",
     formats: ["webp", "jpeg"],
     widths: ["auto"],
     failOnError: false,
+    transformOnRequest: process.env.ELEVENTY_RUN_MODE !== "build",
+    cacheOptions: {
+      duration: process.env.ELEVENTY_RUN_MODE === "build" ? "1d" : "30d",
+    },
+    concurrency: 4,
     defaultAttributes: {
       loading: "lazy",
       decoding: "async",