fix: make source maps opt-in to restore v1.67.0 memory baseline

Production source maps (--enable-source-maps) caused a ~50% memory increase
(600MB to 1000MB) in v1.68.0. Node.js loads all .map files into memory when
this flag is enabled, and Next.js server chunks generate hundreds of map files.

Changes:
- Guard productionBrowserSourceMaps and serverSourceMaps behind
  HOMARR_ENABLE_SOURCE_MAPS env var (default off)
- Guard .map copy step and NODE_OPTIONS in Dockerfile behind
  ENABLE_SOURCE_MAPS build arg (default false)
- run.sh checks HOMARR_ENABLE_SOURCE_MAPS before setting --enable-source-maps
- Add Docker-based memory regression CI job (<700MB threshold)

Closes #6123

apps/nextjs/next.config.ts

@@ -31,8 +31,7 @@ const nextConfig: NextConfig = {
   },
   output: "standalone",
   reactStrictMode: true,
-  // Ship browser source maps so production client stack traces are readable, see https://github.com/homarr-labs/homarr/issues/5891
-  productionBrowserSourceMaps: true,
+  productionBrowserSourceMaps: process.env.HOMARR_ENABLE_SOURCE_MAPS === "true",
   // react compiler breaks mantine-react-table, so disabled for now
   //reactCompiler: true,
   /** We already do typechecking as separate tasks in CI */
@@ -43,8 +42,7 @@ const nextConfig: NextConfig = {
    */
   serverExternalPackages: ["dockerode", "isomorphic-dompurify", "jsdom", "better-sqlite3"],
   experimental: {
-    // Ship server source maps so production Node stack traces in logs reference original source, see https://github.com/homarr-labs/homarr/issues/5891
-    serverSourceMaps: true,
+    serverSourceMaps: process.env.HOMARR_ENABLE_SOURCE_MAPS === "true",
     optimizePackageImports: ["@mantine/core", "@mantine/hooks", "@tabler/icons-react"],
     turbopackFileSystemCacheForDev: true,
     preloadEntriesOnStart: false,