feat: persist chat messages to SQLite

Chat messages were ephemeral — lost on page refresh or late join.
Add a chat_messages table and load the last 50 messages on connect
so players see conversation history.

db/queries/chat.sql

@@ -0,0 +1,9 @@
+-- name: CreateChatMessage :exec
+INSERT INTO chat_messages (game_id, nickname, color, message, created_at)
+VALUES (?, ?, ?, ?, ?);
+
+-- name: GetChatMessages :many
+SELECT * FROM chat_messages
+WHERE game_id = ?
+ORDER BY created_at DESC, id DESC
+LIMIT 50;