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/public/.htaccess

@@ -1,4 +1,9 @@
 RewriteEngine On
+
+# Proxy API requests to api.fahrschultermin.de
+RewriteRule ^api/v1/(.*)$ https://api.fahrschultermin.de/api/v1/$1 [P,L]
+
+# Serve static assets directly
 RewriteCond %{REQUEST_FILENAME} !-f
 RewriteCond %{REQUEST_FILENAME} !-d
-RewriteRule ^ index.php [QSA,L]
+RewriteRule ^ index.php [QSA,L]