Make TRUSTED_PROXIES' insecure-by-default risk loudly visible (#94)

TRUSTED_PROXIES ships empty in .env.example, which disables
AuthMiddleware's reverse-proxy allowlist entirely — a fresh deployment
that doesn't explicitly set it has zero verification that
Remote-User/Remote-Groups headers actually came from the trusted
Authelia proxy. Anything that can reach the app directly (a
misconfigured firewall rule, an exposed container port, SSRF from
another internal service) can set Remote-User: admin and fully
impersonate any user with zero authentication. The enforcement logic
itself was already correct; this was purely a dangerous, easy-to-miss
default.

Added a boxed, unmissable warning around TRUSTED_PROXIES in
.env.example (previously just an inline comment easy to skim past),
added the same warning to README's setup instructions (which didn't
mention this variable at all), and added a Check 8 to api/health.php
that reports a 'warning' status when TRUSTED_PROXIES is empty, so a
deployment that forgets it doesn't go unnoticed after the fact.
Verified against real MariaDB via a running server: the health
endpoint correctly reports 'warning' when empty and 'ok' once set.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lhz7pGMaoTfL5sdYS5XiKv

api/health.php

@@ -162,6 +162,21 @@ if ($maxExecTime === 0 || $maxExecTime >= $requirements['min_max_execution_time'
     ];
 }
 
+// Check 8: TRUSTED_PROXIES configured. Empty disables enforceTrustedProxy()'s
+// allowlist entirely, meaning anything that can reach this app directly can
+// spoof the Authelia forward-auth Remote-* headers and impersonate any user,
+// including an admin. Not fatal (a fresh/dev install may not sit behind a
+// proxy yet), but should never go unnoticed on a real deployment.
+if (!empty($GLOBALS['config']['TRUSTED_PROXIES'] ?? [])) {
+    $checks['trusted_proxies'] = ['status' => 'ok', 'message' => 'configured'];
+} else {
+    $checks['trusted_proxies'] = [
+        'status' => 'warning',
+        'message' => 'TRUSTED_PROXIES is empty — forward-auth headers are NOT verified; '
+            . 'anything that can reach this app directly can impersonate any user'
+    ];
+}
+
 // Calculate response time
 $responseTime = round((microtime(true) - $startTime) * 1000, 2);