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/wait-for-db.sh

@@ -120,12 +120,14 @@ echo "Ensuring storage directories exist..."
 STORAGE_BASE="${STORAGE_PATH:-/app/storage}"
 mkdir -p "$STORAGE_BASE/events/active" "$STORAGE_BASE/events/archived" "$STORAGE_BASE/thumbnails" 2>/dev/null || true
 
-# Run migrations (use safe runner in production)
+# Run migrations (use safe runner in production). Invoked via node directly —
+# the runtime image no longer ships npm (see Dockerfile: its bundled deps kept
+# tripping CVE scanners while npm itself never runs in production).
 echo "Running database migrations..."
 if [ "$NODE_ENV" = "production" ]; then
-  npm run migrate:safe
+  node migrations/run-migrations-safe.js
 else
-  npm run migrate
+  node migrations/run-migrations.js
 fi
 
 # Execute the main command