security: address second-pass audit findings

- CI-C1 false positive: actions/{checkout,setup-node,upload-artifact}
  @v6 do exist (v6.0.0 releases published Oct-Dec 2025). Restore the
  @v6 refs — the earlier batch-1 commit downgraded them unnecessarily.
- Widen idempotency_keys primary key to (key, user_id, method, path)
  via new migration. Batch 1 widened the middleware lookup but left
  the table PK at (key, user_id), so `INSERT OR IGNORE` silently
  skipped the second endpoint that reused a key — the cache was
  never populated for it and a replay re-ran the handler. The
  migration rebuilds the table preserving existing rows (the old
  narrower PK guarantees no conflicts against the new looser key).
- HSTS: keep `includeSubDomains` OFF by default. Enabling it for
  every NODE_ENV=production install would break apex-domain setups
  where siblings still serve HTTP. Operators who want the stricter
  policy opt in with HSTS_INCLUDE_SUBDOMAINS=true.
- Extend the idempotency unit tests to cover the (method, path)
  dimension — same user+key on different path no longer replays.

.github/workflows/test.yml

@@ -17,9 +17,9 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v6
         with:
           node-version: 22
           cache: npm
@@ -33,7 +33,7 @@ jobs:
 
       - name: Upload coverage
         if: success()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: backend-coverage
           path: server/coverage/
@@ -44,9 +44,9 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v6
         with:
           node-version: 22
           cache: npm
@@ -60,7 +60,7 @@ jobs:
 
       - name: Upload coverage
         if: success()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: frontend-coverage
           path: client/coverage/