fix(security): resolve Docker image CVEs for code scanning alerts

- Upgrade nginx base from 1.27-alpine to 1.28-alpine (Alpine 3.23, OpenSSL 3.5.5)
- Upgrade npm to latest in backend production stage to fix tar, minimatch, brace-expansion CVEs
- Add brace-expansion and minimatch overrides for app-level transitive deps
- Remove incompatible body-parser v2 override (breaks Express 4 JSON parsing)
- Remove npm upgrade from builder stages (npm 11 breaks npm ci with existing lockfile)

backend/Dockerfile

@@ -11,10 +11,6 @@ LABEL org.opencontainers.image.source="https://github.com/the-luap/picpeak"
 LABEL org.opencontainers.image.description="PicPeak Backend Service"
 LABEL org.opencontainers.image.licenses="MIT"
 
-# Upgrade npm to fix glob CVE-2025-64756 vulnerability
-# Pin to npm 10.x which supports --omit=dev flag
-RUN npm install -g npm@10
-
 WORKDIR /app
 
 # Copy package files
@@ -34,9 +30,8 @@ WORKDIR /app
 # Upgrade all packages to fix security vulnerabilities (OpenSSL, libexpat, BusyBox CVEs)
 RUN apk upgrade --no-cache
 
-# Upgrade npm to fix glob CVE-2025-64756 vulnerability
-# Pin to npm 10.x which supports --omit=dev flag
-RUN npm install -g npm@10
+# Upgrade npm to latest to fix tar, minimatch, brace-expansion CVEs in npm's own deps
+RUN npm install -g npm@latest
 
 # Install dumb-init for proper signal handling and postgresql-client for database checks
 RUN apk add --no-cache dumb-init postgresql-client