feat: add libSQL/Turso database backend with full feature parity

Introduce a Database trait abstraction (~60 async methods) enabling
compile-time backend selection between PostgreSQL and libSQL/Turso.
Convert all modules from concrete Store to Arc<dyn Database>, add
LibSqlSecretsStore and LibSqlWasmToolStore implementations, wire
libsql stores throughout CLI and main entry points, and make the
setup wizard backend-agnostic.

Key changes:
- src/db/: Database trait, PostgresDatabase adapter, LibSqlBackend
  with native SQLite-dialect SQL, and idempotent migration system
- src/secrets/store.rs: LibSqlSecretsStore (all 8 trait methods)
- src/tools/wasm/storage.rs: LibSqlWasmToolStore (all 7 trait methods)
- src/main.rs, cli/tool.rs, cli/mcp.rs: backend-conditional wiring
- src/setup/channels.rs: SecretsContext uses Arc<dyn SecretsStore>
- Feature-gate postgres-only tests and examples

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

src/setup/mod.rs

@@ -20,6 +20,7 @@
 
 mod channels;
 mod prompts;
+#[cfg(feature = "postgres")]
 mod wizard;
 
 pub use channels::{
@@ -29,4 +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")]
 pub use wizard::{SetupConfig, SetupWizard};