refactor: replace hardcoded credentials with env vars

- Use getenv() with RuntimeException for missing required vars
- Add .env.example template
- Add .env to .gitignore
- Fix missing return statement in config array

config/configuration.local.php

@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * Database configuration - credentials loaded from environment variables
+ */
+
+$getEnvOrThrow = function(string $name): string {
+    $value = getenv($name);
+    if ($value === false || $value === '') {
+        throw new \RuntimeException("Environment variable {$name} is not set");
+    }
+    return $value;
+};
+
+return array('db_req' =>
+    array(
+        'driver_class' => '\PDO',
+        'dsn' => $getEnvOrThrow('DB_DSN'),
+        'driver_params' => array(
+            'username' => $getEnvOrThrow('DB_USER'),
+            'password' => $getEnvOrThrow('DB_PASSWORD'),
+        ),
+    )
+);