fix: use pool instead of single conn for parallel queries

Single MySQL connection cannot handle concurrent prepared statements
via Promise.all — causes protocol conflicts and JSON parse errors.
Switch all bi.queries.js functions to use pool.execute() (each query
gets its own connection from pool). Bump connectionLimit 10→20.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/db-rds.js

@@ -9,7 +9,7 @@ const pool = mysql.createPool({
   password: process.env.PW_MYSQL,
   database: 'cambio_db',
   waitForConnections: true,
-  connectionLimit: 10,
+  connectionLimit: 20,
 });
 
 module.exports = pool;