56bf665397
SQLite engines now use NullPool instead of QueuePool, eliminating the "QueuePool limit of size 5 overflow 10 reached" TimeoutError under concurrent load. PostgreSQL/MySQL engines use a configurable QueuePool with sensible defaults (pool_size=10, max_overflow=20) exposed via DB_POOL_SIZE, DB_MAX_OVERFLOW, DB_POOL_TIMEOUT, DB_POOL_RECYCLE env vars. pool_pre_ping is enabled on all backends. Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>