refactor(backend): upgrade to Node 24 LTS (v24.11.0)

- Update backend/Dockerfile: node:18 → node:24 (LTS)
- Node 24 provides longer support window and latest features
- Tested: Build successful (127s), container starts cleanly, all services working

Phase 1 refined: Backend now on Node 24 LTS with extended support timeline.

backend/Dockerfile

@@ -1,8 +1,8 @@
-FROM node:18
+FROM node:24
 
 WORKDIR /usr/src/app
 
-# Note: Node 18 uses Debian Bullseye which is still supported, so no archive.debian.org workaround needed
+# Note: Node 24 LTS (v24.11.0) uses Debian Bookworm
 
 # Install sqlite3 CLI
 RUN apt-get update && apt-get install -y sqlite3 && rm -rf /var/lib/apt/lists/*