perf: use jemalloc as the default allocator (#1910)

* feat(docker): add jemalloc to reduce memory fragmentation

Install libjemalloc2 in the base image and preload it via LD_PRELOAD in
docker-entrypoint when available. Reduces RSS growth from glibc's default
allocator fragmentation under Rails workloads.

* feat(docker): add DISABLE_JEMALLOC env var + preserve existing LD_PRELOAD

* feat(docker): add jemalloc status logging to entrypoint

* refactor(docker): simplify jemalloc logging to warn-only when disabled/missing

bin/docker-entrypoint

@@ -1,5 +1,15 @@
 #!/bin/bash -e
 
+# Use jemalloc to reduce memory fragmentation if available
+JEMALLOC="/usr/lib/$(uname -m)-linux-gnu/libjemalloc.so.2"
+if [ -f "$JEMALLOC" ] && [ -z "${DISABLE_JEMALLOC}" ]; then
+  export LD_PRELOAD="${LD_PRELOAD:+$LD_PRELOAD:}$JEMALLOC"
+else
+  [ -n "${DISABLE_JEMALLOC}" ] \
+    && echo "WARNING: jemalloc disabled via DISABLE_JEMALLOC" \
+    || echo "WARNING: jemalloc not found at $JEMALLOC, skipping"
+fi
+
 # If running the rails server then create or migrate existing database
 if [ "${1}" == "./bin/rails" ] && [ "${2}" == "server" ]; then
   ./bin/rails db:prepare