feat: enable onboarding wizard for libSQL builds

Refactor the setup wizard to work with both postgres and libsql feature
flags. Previously the wizard was gated behind #[cfg(feature = "postgres")]
only, so libsql-only builds would print an error on `ironclaw onboard`.

- Add libsql fields to Settings (database_backend, libsql_path, libsql_url)
- Split wizard database/migration/secrets methods into feature-gated variants
- Add step_database_libsql() with local path and Turso remote replica prompts
- Update setup/mod.rs and main.rs feature gates to any(postgres, libsql)
- Extend check_onboard_needed() to detect libsql database presence

Co-Authored-By: Claude Opus 4.6 <[email protected]>

src/setup/mod.rs

@@ -20,7 +20,7 @@
 
 mod channels;
 mod prompts;
-#[cfg(feature = "postgres")]
+#[cfg(any(feature = "postgres", feature = "libsql"))]
 mod wizard;
 
 pub use channels::{
@@ -30,5 +30,5 @@ pub use prompts::{
     confirm, input, optional_input, print_error, print_header, print_info, print_step,
     print_success, secret_input, select_many, select_one,
 };
-#[cfg(feature = "postgres")]
+#[cfg(any(feature = "postgres", feature = "libsql"))]
 pub use wizard::{SetupConfig, SetupWizard};