Integrate Monolog: replace four logging systems with single PSR-3 factory

- Add monolog/monolog dependency (^3.10)  
- Create app/Logger.php central factory with channels: app, admin, error, audit
- Each channel gets RotatingFileHandler (30-day retention) with pass-through LineFormatter
  preserving existing JSON format contracts
- Rewrite AppLogger as thin facade delegating to Logger::get('app')
- Rewrite ErrorHandler::log() to delegate to Logger::get('error')
- Rewrite AdminLogger file output to delegate to Logger::get('admin'), keep DB writes
- Add Monolog file shadow to Audit via Logger::get('audit') (Option A per monolog-plan)
- Log level controlled by LOG_LEVEL env var (defaults: DEBUG in cli-server, WARNING otherwise)
- Graceful NullHandler fallback when log directory is not writable
- Update SystemController LOG_FILES: remove php_error, add app/admin/error/audit
- JSON app logs parsed to readable one-liners in the log viewer
- Remove nginx config tab (parametres + fragment + template + css)
- Friendly empty-state message when app log files don't exist yet (notYet)
- PHP tail fallback when exec() unavailable
- All 228 PHPUnit tests pass, no call sites changed

app/src/Logger.php

@@ -0,0 +1,105 @@
+<?php
+
+use Monolog\Handler\RotatingFileHandler;
+use Monolog\Handler\NullHandler;
+use Monolog\Formatter\LineFormatter;
+use Monolog\Level;
+use Psr\Log\LoggerInterface;
+
+/**
+ * Central logger factory — provides named Monolog channel instances
+ * backed by rotating JSON-line log files.
+ *
+ * Channels:
+ *  - 'app'   → replaces AppLogger
+ *  - 'admin' → replaces AdminLogger file output
+ *  - 'error' → replaces ErrorHandler logging
+ *  - 'audit' → replaces Audit file shadow (DB writes stay in Audit)
+ *
+ * Usage:
+ *   Logger::get('app')->info('message', [...]);
+ *   Logger::get('admin')->warning('message', [...]);
+ */
+class Logger
+{
+    /** @var array<string, LoggerInterface> */
+    private static array $channels = [];
+
+    /**
+     * Get (or lazily create) a named Monolog channel.
+     */
+    public static function get(string $channel): LoggerInterface
+    {
+        if (!isset(self::$channels[$channel])) {
+            self::$channels[$channel] = self::create($channel);
+        }
+        return self::$channels[$channel];
+    }
+
+    /**
+     * Create a Monolog channel with rotating JSON file handler.
+     *
+     * Falls back to NullHandler if the log directory is not writable
+     * (e.g. CLI scripts on a machine that doesn't have the production path).
+     */
+    private static function create(string $channel): \Monolog\Logger
+    {
+        $logDir = self::logDir();
+
+        if (!is_dir($logDir) && !@mkdir($logDir, 0755, true) && !is_dir($logDir)) {
+            // Directory can't be created — use null handler (graceful degradation)
+            $logger = new \Monolog\Logger($channel);
+            $logger->pushHandler(new NullHandler());
+            return $logger;
+        }
+
+        try {
+            $handler = new RotatingFileHandler(
+                $logDir . '/' . $channel . '.log',
+                30,          // keep 30 days of logs
+                self::level()
+            );
+        } catch (\Throwable $e) {
+            // Can't open log file — use null handler
+            $logger = new \Monolog\Logger($channel);
+            $logger->pushHandler(new NullHandler());
+            return $logger;
+        }
+
+        // Pass-through formatter: the facades (AppLogger, AdminLogger, etc.)
+        // construct their own JSON lines and pass them as the log message.
+        // %message% preserves the existing JSON format contract exactly.
+        $handler->setFormatter(new LineFormatter("%message%\n", null, true));
+
+        $logger = new \Monolog\Logger($channel);
+        $logger->pushHandler($handler);
+
+        return $logger;
+    }
+
+    /**
+     * Read the LOG_LEVEL env var with sensible defaults.
+     */
+    private static function level(): Level
+    {
+        $level = strtoupper(getenv('LOG_LEVEL') ?: '');
+
+        // Default: WARNING in production (always set in .env), DEBUG otherwise
+        if ($level === '') {
+            return php_sapi_name() === 'cli-server' ? Level::Debug : Level::Warning;
+        }
+
+        return Level::fromName($level);
+    }
+
+    /**
+     * Resolve the log directory.
+     */
+    private static function logDir(): string
+    {
+        if (defined('STORAGE_ROOT')) {
+            return STORAGE_ROOT . '/logs';
+        }
+        return __DIR__ . '/../storage/logs';
+    }
+}