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

scripts/run.sh

@@ -58,6 +58,12 @@ terminate() {
 
 trap terminate TERM INT
 
+# Enable source maps in Node.js when explicitly opted in.
+# Default off — see https://github.com/homarr-labs/homarr/issues/6123
+if [ "${HOMARR_ENABLE_SOURCE_MAPS}" = "true" ]; then
+  export NODE_OPTIONS="--enable-source-maps${NODE_OPTIONS:+ $NODE_OPTIONS}"
+fi
+
 node apps/nextjs/server.js &
 NEXTJS_PID=$!