feat: scaffold klarbild foundation (astro+postgres+s3, auth, migrations, health)

- Astro 5 SSR (node standalone) + React, OKLCH tokens (no tailwind)
- migrations/001_init.sql: full schema per 03-datenmodell-api
- lib: db+migrations, crypto (AES-256-GCM), auth (argon2+signed session, ratelimit),
  storage (S3/MinIO, presigned URLs), openrouter (POST /v1/images, cost)
- middleware: init-once + session guard + admin gate; /api/health (db+storage)
- login + studio placeholder; seeds (till/lea, default recipes); Dockerfile
- verified: astro build passes

.gitignore

@@ -0,0 +1,6 @@
+.env
+node_modules
+dist
+.astro
+*.log
+uploads/