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>

eslint.config.mjs

@@ -7,7 +7,7 @@ const eslintConfig = defineConfig([
   ...nextVitals,
   ...nextTs,
   prettier,
-  globalIgnores([".next/**", "out/**", "build/**", "coverage/**", "next-env.d.ts"]),
+  globalIgnores([".next/**", "out/**", "build/**", "coverage/**", "next-env.d.ts", "src/generated/**"]),
 ]);
 
 export default eslintConfig;