ci: harden workflow + add security scanning and format gates

- permissions: contents:read (least privilege), concurrency cancel,
  scope push to main+tags (was: every branch, double-running with PRs),
  per-job timeout-minutes.
- security: npm audit --omit=dev, CodeQL SAST (TS + C#), Dependabot
  (npm/nuget/actions).
- format: npm run format:check + dotnet format --verify-no-changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,17 @@
+version: 2
+updates:
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 10
+
+  - package-ecosystem: nuget
+    directory: /backend
+    schedule:
+      interval: weekly
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly