Perf: Increase database connection pool size (#7559)

### What problem does this PR solve?

1. The MySQL instance is configured with max_connections=1000,
but our connection pool was limited to max_connections: 100.
This mismatch caused connection pool exhaustion during performance
testing.

2.  Increase stale_timeout to resolve #6548

### Type of change

- [x] Performance Improvement

docker/service_conf.yaml.template

@@ -7,8 +7,8 @@ mysql:
   password: '${MYSQL_PASSWORD:-infini_rag_flow}'
   host: '${MYSQL_HOST:-mysql}'
   port: 3306
-  max_connections: 100
-  stale_timeout: 30
+  max_connections: 900
+  stale_timeout: 300
 minio:
   user: '${MINIO_USER:-rag_flow}'
   password: '${MINIO_PASSWORD:-infini_rag_flow}'