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/Cargo.lock

@@ -771,6 +771,7 @@ dependencies = [
  "proptest",
  "prost-types",
  "rand 0.10.1",
+ "rand_core 0.10.1",
  "rcgen",
  "restructed",
  "rstest",