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/BaseRepository.php

@@ -16,6 +16,11 @@ abstract class BaseRepository
         $this->db = Database::connection();
     }
 
+    protected function lastInsertId(): string
+    {
+        return Database::lastInsertId();
+    }
+
     protected function now(): string
     {
         return gmdate('c');