fix(webdav): constant-time compare on lock-token equality checks

Replace plain `==` on lock tokens with `subtle::ConstantTimeEq` at
every token-comparison site on the WebDAV surface. Closes a
reported timing side-channel (2026-09-05) in `evaluate_if_header`
where an authenticated attacker could theoretically recover another
user's active lock token via response-latency measurements on the
`If:` header state-token comparison.

Practical exploitability is marginal — the signal is tens-of-ns
buried under ms-scale network jitter, ~5×10⁸ samples needed per
token to average through the noise vs a default lock lifetime of
60 s to 1 h — but the fix is a five-line change with zero
measurable perf cost (`subtle` is already transitive via
sqlx-postgres → digest, so no new binary weight), and adopting
constant-time compare on any token that gates access matches the
hygiene rule the rest of the codebase already follows on password
and session paths.

Sites fixed:
* `evaluate_if_header` — first-pass state-token scan and
  second-pass condition eval in `webdav_handler.rs`.
* `WebdavLockService::refresh` — `!= token` mismatch check.
* `WebdavLockService::release` — `== token` guard on the
  by_path invalidation branch.

The two `WebdavLockService` sites are already gated by
`self.by_token.get(token)?` — the attacker cannot reach the
comparison without already presenting a valid token, so their
timing surface is nil in practice. Kept constant-time anyway for
callsite consistency.

Sweep confirmed no other secret-adjacent `==` in production code:
password verification goes through Argon2's `verify_password`,
session/CSRF/DPoP jti tokens are hashmap-gated, and blob-hash
equality compares two server-side values with no attacker-
controlled operand.

Reported-by: Abdurazzoqov Javohir <abdurazzoqovjavohir700-dev@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.lock

@@ -4705,6 +4705,7 @@ dependencies = [
  "smol_str",
  "socket2 0.6.4",
  "sqlx",
+ "subtle",
  "tantivy",
  "tempfile",
  "testcontainers-modules",