add balance/mean-reversion signal and Cloudflare visitor logging

Balance signal (15% weight) favors under-represented chairs over last 50
games. Visitor middleware captures real IPs from CF headers, batched into
ClickHouse with 90-day TTL.

clickhouse/init.sql

@@ -40,3 +40,16 @@ CREATE TABLE IF NOT EXISTS users (
     updated_at    DateTime DEFAULT now()
 ) ENGINE = ReplacingMergeTree(updated_at)
 ORDER BY user_id;
+
+CREATE TABLE IF NOT EXISTS visitors (
+    ip            String,
+    country       String,
+    path          String,
+    method        String,
+    user_agent    String,
+    referer       String,
+    accept_lang   String,
+    created_at    DateTime DEFAULT now()
+) ENGINE = MergeTree()
+ORDER BY (created_at, ip)
+TTL created_at + INTERVAL 90 DAY;