feat(auth): ALLOW_SELF_REGISTER toggle + public /auth/config endpoint (0.42.0)

Lets operators run a closed-membership deployment by setting
ALLOW_SELF_REGISTER=false (default true, so existing deploys are
unaffected). When off, POST /auth/register returns 403 forbidden. The
rate-limit token is consumed BEFORE the disabled check so the timing
doesn't distinguish enabled-but-rejected from disabled — closes the
toggle-state probe channel.

New public GET /auth/config returns { self_register_enabled: bool }
so the frontend can render its register affordances correctly
without conflating "disabled" with "rate-limited" (which a probe
attempt would).

Frontend: a lightweight reactive `authConfig` store loads the flag
once on root-layout mount (and again on /register direct navigation,
which bypasses the layout's onMount). Header hides the Register link
when the toggle is off; /register renders a "self-registration is
disabled — ask an administrator" notice instead of the form.

Admin-create endpoint that pairs with this toggle is intentionally
not in this PR — it lands as the next branch (feat/admin-user-create).
The toggle alone is independently useful for deployments that want
to lock down enrollment without yet wiring an admin UI.

backend/Cargo.lock

@@ -1470,7 +1470,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.41.2"
+version = "0.42.0"
 dependencies = [
  "anyhow",
  "argon2",