v0.1.0-layerbase (file persistence)

Fork of PostHog/duckgres with opt-in file persistence.

Changes from upstream:

• Added file_persistence config option (YAML, env var, CLI flag)
• When enabled, DuckDB databases persist to <data_dir>/<username>.duckdb
• Default behavior unchanged (in-memory per connection)
• Backward-compatible with upstream duckgres configs