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/src/auth/token.rs

@@ -3,15 +3,16 @@
 //! `generate_token` draws 32 bytes from the OS CSPRNG, encodes them as
 //! URL-safe base64 (no padding), and returns the raw string alongside its
 //! SHA-256 hash. Storage holds only the hash; the raw value lives in the
-//! cookie or `Authorization` header. Comparison goes through
-//! `constant_time_eq` to keep timing side channels off the table.
+//! cookie or `Authorization` header. Token lookup is an indexed equality on
+//! that 256-bit hash in the database (`WHERE token_hash = $1`), so there's no
+//! in-process secret comparison to time-attack: a guess has to match a full
+//! SHA-256 digest, and the DB index reveals nothing about how close it came.
 
 use base64::engine::general_purpose::URL_SAFE_NO_PAD;
 use base64::Engine as _;
 use rand::rngs::OsRng;
 use rand::RngCore;
 use sha2::{Digest, Sha256};
-use subtle::ConstantTimeEq;
 
 pub const TOKEN_BYTES: usize = 32;
 pub const HASH_BYTES: usize = 32;
@@ -30,10 +31,6 @@ pub fn hash_token(raw: &str) -> [u8; HASH_BYTES] {
     hasher.finalize().into()
 }
 
-pub fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
-    a.ct_eq(b).into()
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -58,11 +55,4 @@ mod tests {
         assert_eq!(hash_token("abc"), hash_token("abc"));
         assert_ne!(hash_token("abc"), hash_token("abd"));
     }
-
-    #[test]
-    fn constant_time_eq_compares_correctly() {
-        assert!(constant_time_eq(b"abc", b"abc"));
-        assert!(!constant_time_eq(b"abc", b"abd"));
-        assert!(!constant_time_eq(b"abc", b"abcd"));
-    }
 }