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/secrets/mod.rs

@@ -64,7 +64,11 @@ mod store;
 mod types;
 
 pub use crypto::SecretsCrypto;
-pub use store::{PostgresSecretsStore, SecretsStore};
+#[cfg(feature = "libsql")]
+pub use store::LibSqlSecretsStore;
+#[cfg(feature = "postgres")]
+pub use store::PostgresSecretsStore;
+pub use store::SecretsStore;
 pub use types::{
     CreateSecretParams, CredentialLocation, CredentialMapping, DecryptedSecret, Secret,
     SecretError, SecretRef,