Harden bearer auth: constant-time compare, hashed at rest, authoritative actor_id

Fixes two live authz bugs in omnigraph-server:

- Bearer-token lookup previously used HashMap::get, which compares keys with
  Eq and short-circuits on the first differing byte — a network-observable
  timing oracle for brute-forcing tokens. Tokens are now stored as SHA-256
  digests and compared with subtle::ConstantTimeEq, iterating every entry
  unconditionally so total work is independent of which slot matches. Raw
  token bytes no longer live in server memory after startup.

- authorize_request now overwrites PolicyRequest.actor_id from the
  authenticated session instead of trusting the handler-supplied field,
  which previously defaulted to "" via unwrap_or_default(). The empty
  string can no longer reach Cedar as a policy subject even if a future
  refactor drops the None check.

External API of AppState constructors is unchanged — tokens still enter as
Vec<(String, String)> and are hashed on the way in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -66,6 +66,7 @@ utoipa = { version = "5", features = ["axum_extras"] }
 url = "2"
 cedar-policy = "4.9"
 sha2 = "0.10"
+subtle = "2"
 
 [profile.dev]
 debug = 0