Function init_schema 

pub fn init_schema(
    path: &Path,
    schema_ddl: &str,
    expected_version: u32,
) -> Result<Connection>

Initialize a SQLite database with WAL mode, foreign keys, and a custom schema.

Creates parent directories if needed. Applies standard PRAGMAs for safety and performance. Runs the provided DDL to create tables, then validates the schema version matches expectations.