🐛🧪 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.

Cargo.lock

@@ -2107,6 +2107,7 @@ name = "vanth_cli"
 version = "0.1.0"
 dependencies = [
  "clap",
+ "serde",
  "serde_json",
  "tempfile",
  "tracing",