fix(security): close 5 Trivy alerts — postcss/tar bumps + drop npm from the runtime image (stable) (#879)

* 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. (stable)

* 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. (stable)

backend/__tests__/integration/restoreService.pgBranch.test.js

@@ -183,22 +183,24 @@ describe('restoreService — PG branch scope contract (PR #596 review)', () => {
     expect(window).toMatch(/was_successful:\s*true/);
   });
 
-  it('npm run migrate:safe is invoked after the replay in restore()', () => {
+  it('the safe migration runner is invoked after the replay in restore()', () => {
     // Contract from PR #596 round 4: backups taken on older picpeak
     // versions must restore COMPLETELY on a newer image — even if new
     // migrations have been added since the backup was taken. The
-    // restore() flow shells out to `npm run migrate:safe` AFTER the
+    // restore() flow shells out to the safe migration runner AFTER the
     // operator-meta replay so the schema catches up to the running
     // code WITHIN the restore boundary (not on the next container
-    // restart).
+    // restart). Invoked as `node migrations/run-migrations-safe.js` —
+    // the runtime image ships no npm, so the former `npm run
+    // migrate:safe` would ENOENT into the non-fatal catch.
     //
     // Contract:
-    //   1. A `migrate:safe` shell-out exists somewhere in restoreService
+    //   1. A run-migrations-safe shell-out exists somewhere in restoreService
     //   2. It sits AFTER the replay drain — verification → replay →
     //      migrations is the documented order
     //   3. It does NOT sit inside performDatabaseRestore (must run
     //      against the reinit'd pool from the parent restore())
-    const migrateLine = findFirst(/['"]migrate:safe['"]/);
+    const migrateLine = findFirst(/run-migrations-safe\.js/);
     expect(migrateLine).toBeGreaterThan(0);
 
     const replayLine = findLast(/this\.preservedMetaSnapshot\.length\s*>\s*0/);