fix(security): close 5 Trivy alerts — postcss/tar bumps + drop npm from the runtime image (#878)

* fix(security): close the 5 open Trivy alerts — dep bumps + drop npm from the runtime image

Backend deps:
- postcss 8.5.10 -> 8.5.18 (CVE-2026-45623, GHSA-r28c-9q8g-f849; the pin
  exists to force sanitize-html's transitive copy onto a fixed version)
- tar pin/override >=7.5.16 -> >=7.5.21, resolves 7.5.22
  (GHSA-r292-9mhp-454m)

Runtime image:
- Remove the npm CLI from the final stage instead of upgrading it: npm's
  bundled node_modules ship tar 7.5.19 and brace-expansion 5.0.7 (no npm
  release bundles the fixed versions — checked 11.18.0 and 12.0.1), and
  npm never runs in production. wait-for-db.sh now invokes the migration
  runners via node directly. This ends the recurring npm-bundled-CVE
  alert class; the previous 'npm install -g npm@11' line was itself a
  patch for the last batch.

* fix(restore): run post-restore migrations via node — the image ships no npm

restoreService still shelled out to 'npm run migrate:safe' after a
restore; with npm removed from the runtime image that would ENOENT into
the non-fatal catch, silently leaving a restored older backup on a
schema behind the running code until the next container restart. Invoke
migrations/run-migrations-safe.js through node directly, matching
wait-for-db.sh. The PR #596 source-contract test now pins the new
invocation.

backend/Dockerfile

@@ -37,14 +37,16 @@ ARG CACHEBUST=1
 # Upgrade all packages to fix security vulnerabilities (OpenSSL, libexpat, BusyBox CVEs)
 RUN echo "cachebust=${CACHEBUST}" && apk upgrade --no-cache
 
-# Upgrade the npm CLI in the final image so its bundled deps are patched
-# (sigstore 4.x, tar) — closes CVE-2026-48815 and the older @sigstore/core / tar
-# Trivy alerts. Safe here: only the CLI present in the image changes. Runtime
-# dependencies come from the builder stage (COPY --from=builder node_modules
-# below) and the entrypoint runs node, not npm — so npm 11's install behaviour
-# (the reason 10.x was pinned) never executes in this stage. npm 11 needs
-# Node >=22.9, satisfied by node:22-alpine.
-RUN npm install -g npm@11
+# Remove the npm CLI from the final image. Nothing runs npm here: the
+# entrypoint is node, runtime deps are COPY'd from the builder stage, and
+# wait-for-db.sh invokes the migration runners via node directly. npm's
+# bundled node_modules kept tripping Trivy (sigstore, tar 7.5.19,
+# brace-expansion 5.0.7 — even npm 12.0.1 still ships the vulnerable
+# copies), so shipping no npm ends that alert class instead of chasing
+# per-release patches. Note: `docker exec … npm run <script>` no longer
+# works in the container — use `node migrations/run-migrations-safe.js`
+# and friends instead.
+RUN rm -rf /usr/local/lib/node_modules/npm /usr/local/bin/npm /usr/local/bin/npx
 
 # Install dumb-init for proper signal handling, postgresql-client for database
 # checks, ffmpeg for video upload support, and su-exec for the root → nodejs