add nix development environment

Provide a direnv-loaded Laravel and future Vue toolchain with isolated worktree ports and a process-compose stack. Use PostgreSQL for local runtime services while retaining in-memory SQLite for PHPUnit.

backend/config/queue.php

@@ -103,7 +103,7 @@ return [
     */
 
     'batching' => [
-        'database' => env('DB_CONNECTION', 'sqlite'),
+        'database' => env('DB_CONNECTION', 'pgsql'),
         'table' => 'job_batches',
     ],
 
@@ -122,7 +122,7 @@ return [
 
     'failed' => [
         'driver' => env('QUEUE_FAILED_DRIVER', 'database-uuids'),
-        'database' => env('DB_CONNECTION', 'sqlite'),
+        'database' => env('DB_CONNECTION', 'pgsql'),
         'table' => 'failed_jobs',
     ],