Add test and security baseline

- make quick/check/mutation from cj/security-baseline (Makefile unchanged,
  settings in baseline.mk), pre-commit, gitleaks, Semgrep rules, ESLint
  security rules, Forgejo workflow
- tests/security: generic baseline tests plus app config and role tests
  against createApp() with an in-memory database
- extract createApp() from src/server/index.ts; LB_DATABASE_PATH for tests
- fix gaps found by the tests: reject unknown fields on project create,
  answer malformed JSON with 400 instead of 500, set nosniff on the API
- npm audit fix (next 16.4.1 and others, no high/critical left)
- clear ESLint errors, safeFileName without control-character regex
- per-project ownership stays an open, documented gap (idorGap -> TODO)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitleaks.toml

@@ -0,0 +1,3 @@
+# gitleaks: Standardregeln. Ausnahmen nur mit Begründung eintragen.
+[extend]
+useDefault = true