feat(db): Datenmodell, erste Migration und Seed (#003)

Prisma 7.10 mit Postgres-Adapter, 23 Modelle inklusive Rechnungen mit Positionen,
Seed fuer Administrator, Warnregeln und Demo-Daten, Testdatenbank fuer Vitest.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

package.json

@@ -10,9 +10,19 @@
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
-    "format": "prettier --write ."
+    "format": "prettier --write .",
+    "db:up": "docker compose -f docker-compose.dev.yml up -d",
+    "db:down": "docker compose -f docker-compose.dev.yml down",
+    "db:migrate": "prisma migrate dev",
+    "db:seed": "prisma db seed",
+    "db:studio": "prisma studio",
+    "db:generate": "prisma generate",
+    "postinstall": "prisma generate"
   },
   "dependencies": {
+    "@node-rs/argon2": "^2.2.1",
+    "@prisma/adapter-pg": "^7.10.0",
+    "@prisma/client": "^7.10.0",
     "admin-lte": "^4.9.1",
     "bootstrap": "^5.3.8",
     "bootstrap-icons": "^1.13.1",
@@ -25,10 +35,13 @@
     "@types/node": "^24.13.5",
     "@types/react": "^19",
     "@types/react-dom": "^19",
+    "dotenv": "^17.4.2",
     "eslint": "^9",
     "eslint-config-next": "16.3.5",
     "eslint-config-prettier": "^10.1.8",
     "prettier": "^3.9.7",
+    "prisma": "^7.10.0",
+    "tsx": "^4.23.13",
     "typescript": "^5",
     "vitest": "^5.0.1"
   },