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/package.json

@@ -57,7 +57,7 @@
     "pdf-lib": "^1.17.1",
     "pdfkit": "^0.17.2",
     "pg": "^8.16.3",
-    "postcss": "8.5.10",
+    "postcss": "8.5.18",
     "qrcode": "^1.5.4",
     "react-i18next": "^15.6.0",
     "sanitize-html": "^2.17.0",
@@ -66,7 +66,7 @@
     "swagger-jsdoc": "^6.2.8",
     "swagger-ui-express": "^5.0.1",
     "swissqrbill": "^4.3.0",
-    "tar": ">=7.5.16",
+    "tar": ">=7.5.21",
     "uuid": "^11.1.1",
     "winston": "^3.8.2",
     "zxcvbn": "^4.4.2"
@@ -86,7 +86,7 @@
     "js-yaml": "^4.2.0",
     "fast-xml-parser": ">=5.7.0",
     "qs": ">=6.15.2",
-    "tar": ">=7.5.16",
+    "tar": ">=7.5.21",
     "brace-expansion": ">=5.0.7",
     "minimatch": ">=9.0.7",
     "path-to-regexp": "0.1.13",