refactor(logging): replace console.* with structured logger across server

Add server/logger.js - zero-dependency, level-based logger that outputs
JSON in production and human-readable format in development. Controlled
via LOG_LEVEL env var (debug/info/warn/error, default: info).

Replaces all 100 console.log/warn/error calls in 14 server files.

server/logger.js

@@ -0,0 +1,40 @@
+/**
+ * Modul: Logger
+ * Zweck: Levelbasiertes strukturiertes Logging ohne externe Dependencies.
+ *        Ausgabe als JSON in Production, lesbar in Development.
+ * Steuerung: LOG_LEVEL env var (debug, info, warn, error). Default: info.
+ */
+
+'use strict';
+
+const LEVELS = { debug: 0, info: 1, warn: 2, error: 3 };
+const currentLevel = LEVELS[process.env.LOG_LEVEL] ?? LEVELS.info;
+const isProduction = process.env.NODE_ENV === 'production';
+
+function emit(level, mod, msg, extra) {
+  if (LEVELS[level] < currentLevel) return;
+
+  if (isProduction) {
+    const entry = { ts: new Date().toISOString(), level, mod, msg };
+    if (extra !== undefined) entry.extra = extra;
+    process.stdout.write(JSON.stringify(entry) + '\n');
+  } else {
+    const prefix = `[${mod}]`;
+    if (extra !== undefined) {
+      console[level === 'debug' ? 'log' : level](prefix, msg, extra);
+    } else {
+      console[level === 'debug' ? 'log' : level](prefix, msg);
+    }
+  }
+}
+
+function createLogger(mod) {
+  return {
+    debug: (msg, extra) => emit('debug', mod, msg, extra),
+    info:  (msg, extra) => emit('info',  mod, msg, extra),
+    warn:  (msg, extra) => emit('warn',  mod, msg, extra),
+    error: (msg, extra) => emit('error', mod, msg, extra),
+  };
+}
+
+module.exports = { createLogger };