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.toml

@@ -21,6 +21,7 @@ mutants = "0.0.4"
 prost = "0.14.3"
 prost-types = { version = "0.14.3", features = ["chrono"] }
 rand = "0.10.1"
+rand_core = "0.10.1"
 rcgen = { version = "0.14.7", features = [ "aws_lc_rs",  "pem", "x509-parser", "zeroize" ], default-features = false }
 rstest = "0.26.1"
 rustls = { version = "0.23.40", features = ["aws-lc-rs", "logging", "prefer-post-quantum", "std"], default-features = false }