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.

.env.example

@@ -4,6 +4,7 @@
 # Server
 PORT=3000
 NODE_ENV=production
+# LOG_LEVEL=info   # debug, info, warn, error (default: info)
 
 # Session
 SESSION_SECRET=REPLACE_WITH_A_LONG_RANDOM_STRING