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/ErrorHandler.php

@@ -154,7 +154,7 @@ class ErrorHandler
     }
 
     /**
-     * Write a structured error log entry.
+     * Write a structured error log entry — delegates to Monolog 'error' channel.
      *
      * @param string    $context  e.g. 'thesis_edit', 'partage_submit', 'csv_import'
      * @param \Throwable $e
@@ -162,22 +162,22 @@ class ErrorHandler
      */
     public static function log(string $context, \Throwable $e, array $extra = []): void
     {
-        $parts = [
-            'context=' . $context,
-            'exception=' . get_class($e),
-            'message=' . $e->getMessage(),
+        $entry = [
+            'timestamp' => date('c'),
+            'ip'        => $_SERVER['REMOTE_ADDR'] ?? 'unknown',
+            'user_agent' => $_SERVER['HTTP_USER_AGENT'] ?? '',
+            'context'   => $context,
+            'exception' => get_class($e),
+            'message'   => $e->getMessage(),
+            'trace'     => $e->getTraceAsString(),
         ];
-        foreach ($extra as $k => $v) {
-            if (is_scalar($v) || $v === null) {
-                $parts[] = $k . '=' . json_encode($v, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
-            } elseif (is_array($v)) {
-                $parts[] = $k . '=' . json_encode($v, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
-            } else {
-                $parts[] = $k . '=' . gettype($v);
-            }
-        }
-        $parts[] = 'trace=' . $e->getTraceAsString();
 
-        error_log(implode(' | ', $parts));
+        if (!empty($extra)) {
+            $entry['extra'] = $extra;
+        }
+
+        $line = json_encode($entry, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
+
+        Logger::get('error')->error($line);
     }
 }