security(server): use SysRng directly for bootstrap token generation

Replace make_rng()/StdRng with UnwrapErr(SysRng) to eliminate the PRNG
intermediate layer and make OS entropy derivation explicit and unambiguous.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/crates/arbiter-server/Cargo.toml

@@ -31,6 +31,7 @@ diesel_migrations = { version = "2.3.2", features = ["sqlite"] }
 async-trait.workspace = true
 tokio-stream.workspace = true
 rand.workspace = true
+rand_core.workspace = true
 rcgen.workspace = true
 chrono.workspace = true
 kameo.workspace = true