refactor: remove unused constant_time_eq and correct the token doc

constant_time_eq (and the `subtle` dependency) was never wired into any
comparison — the module doc wrongly claimed token comparison used it, but lookup
is an indexed DB equality on the 256-bit SHA-256 hash, so there's no in-process
secret compare to time-attack. Delete the dead fn, its test, and the unused
`subtle` dep; rewrite the doc. No behaviour change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Cargo.toml

@@ -35,7 +35,6 @@ dotenvy = "0.15"
 argon2 = "0.5"
 rand = "0.8"
 sha2 = "0.10"
-subtle = "2"
 base64 = "0.22"
 # Image decode + downscale for the analysis worker (keep the page image
 # under the local vision model's token budget). Only the manga page formats.