feat(backend): upgrade Node 14 → 18 (Phase 1)

- Update backend/Dockerfile: FROM node:14 → node:18
- Remove Debian Buster EOL repository workarounds (Node 18 uses Bullseye)
- Tested: Docker build successful, container starts cleanly, DB initializes

Phase 1 complete: Backend runtime now on Node 18 LTS.

backend/Dockerfile

@@ -1,11 +1,8 @@
-FROM node:14
+FROM node:18
 
 WORKDIR /usr/src/app
 
-# Fix Debian Buster repositories (EOL)
-RUN sed -i 's/deb.debian.org/archive.debian.org/g' /etc/apt/sources.list && \
-    sed -i 's/security.debian.org/archive.debian.org/g' /etc/apt/sources.list && \
-    sed -i '/stretch-updates/d' /etc/apt/sources.list
+# Note: Node 18 uses Debian Bullseye which is still supported, so no archive.debian.org workaround needed
 
 # Install sqlite3 CLI
 RUN apt-get update && apt-get install -y sqlite3 && rm -rf /var/lib/apt/lists/*