Read-only check to verify if viewer has purchased article_id.
fn has_access(
env: soroban_sdk::Env,
viewer: soroban_sdk::Address,
article_id: soroban_sdk::Symbol,
) -> boolUnlocks content by transferring price XLM/token from viewer to creator
and saving the access record in persistent storage.
fn unlock_content(
env: soroban_sdk::Env,
viewer: soroban_sdk::Address,
creator: soroban_sdk::Address,
token: soroban_sdk::Address,
price: i128,
article_id: soroban_sdk::Symbol,
)