🐛🧪 cli, vanth: Fix SQLite store bug and add CLI tests

- Modified `Sqlite::get_all_of_ty` to return stored rows instead of empty results.
- Added `Send` requirement to `Backend` trait.
- Implemented `ContentHash::hex` method for hexadecimal representation.
- Added integration test for CLI `write` and `get` commands with `run_vanth` helper.
- Configured CLI tracing to log to stderr instead of stdout.
- Added `serde` dependency to `cli` crate for test struct serialization.
- Modified `handle_write` to print content hash after successful storage.

crates/cli/Cargo.toml

@@ -5,6 +5,7 @@ edition.workspace = true
 
 [dependencies]
 clap.workspace = true
+serde.workspace = true
 serde_json.workspace = true
 vanth = { path = "../vanth" }
 tracing.workspace = true