Split API and frontend, migrate SQLite to PostgreSQL

- api.fahrschultermin.de: full API entry point with CORS + session auth
- fahrschultermin.de: frontend only, no more /api/v1 routing
- Database.php: PostgreSQL support (pgsql driver + lastval())
- All repositories: use Database::lastInsertId() for PG compatibility
- config/app.php: PostgreSQL connection settings + CORS config
- bootstrap.php: pass full $config to Database::configure()
- public/index.php: fetch-patch to redirect /api/v1 to api.fahrschultermin.de
- database/schema_pgsql.sql: full PostgreSQL schema (BIGSERIAL)
- database/import_pgsql.sql: 518 rows migrated from SQLite

www/fahrschultermin.de/app/Repositories/InstructorRepository.php

@@ -45,7 +45,7 @@ final class InstructorRepository extends BaseRepository
             'updated_at' => $timestamp,
         ]);
 
-        return $this->find($tenantId, (int) $this->db->lastInsertId());
+        return $this->find($tenantId, (int) $this->lastInsertId());
     }
 
     public function update(int $tenantId, int $id, array $data): ?array