Add trusted-proxy auth hardening + PHP requirements checks

Trusted-proxy hardening (defense-in-depth for Authelia forward-auth):
- AuthMiddleware now only honors Remote-* identity headers when REMOTE_ADDR
  is in a configured TRUSTED_PROXIES allowlist; otherwise it refuses with 403
  and logs an 'untrusted_proxy' security event. Previously anything that could
  reach PHP directly could spoof Remote-User/Remote-Groups and log in as admin.
- New config TRUSTED_PROXIES (comma-separated, from .env). Empty = enforcement
  off, so this is backward compatible until the allowlist is set on a host.

Requirements checks (so a PHP upgrade dropping an extension can't silently
break features like avatars again):
- config/requirements.php: single source of truth for min PHP version and
  required extensions (ldap, mysqli, curl, mbstring, fileinfo, json).
- scripts/check_requirements.php: CI script that fails the build if the
  environment doesn't satisfy them.
- New 'requirements' CI job installs those extensions and runs the check;
  deploy now depends on it.
- api/health.php: adds php_extensions + php_version checks so production
  monitoring surfaces the drift (returns 503 if a required extension is gone).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

api/health.php

@@ -95,6 +95,40 @@ if (is_dir($rateLimitDir) && is_writable($rateLimitDir)) {
     ];
 }
 
+// Check 5: Required PHP extensions (catches e.g. a PHP upgrade silently
+// dropping php-ldap, which breaks avatars with no other visible error).
+$requirements = require dirname(__DIR__) . '/config/requirements.php';
+$missingExt = array_values(array_filter(
+    $requirements['required_extensions'],
+    fn($ext) => !extension_loaded($ext)
+));
+if (empty($missingExt)) {
+    $checks['php_extensions'] = [
+        'status' => 'ok',
+        'message' => 'All required extensions loaded'
+    ];
+} else {
+    $checks['php_extensions'] = [
+        'status' => 'error',
+        'message' => 'Missing extensions: ' . implode(', ', $missingExt)
+    ];
+    $healthy = false;
+}
+
+// Check 6: PHP version meets the declared minimum
+if (version_compare(PHP_VERSION, $requirements['min_php_version'], '>=')) {
+    $checks['php_version'] = [
+        'status' => 'ok',
+        'message' => PHP_VERSION
+    ];
+} else {
+    $checks['php_version'] = [
+        'status' => 'error',
+        'message' => sprintf('PHP %s < required %s', PHP_VERSION, $requirements['min_php_version'])
+    ];
+    $healthy = false;
+}
+
 // Calculate response time
 $responseTime = round((microtime(true) - $startTime) * 1000, 2);