Sw/fix template app

[cooleel]

Changes

• Scaffolded app code now emits f"Hello, {name}!" instead of the escaped {{name}} (Rust template double-brace was leaking into generated Python).
• README curl example now uses --json '"World"' to match the SDK-generated command — correct JSON content type and a valid JSON-encoded body (the old -d 'World' sent form-encoded, invalid JSON; -X POST was redundant).