fix: recover the auth rate limiter from a poisoned mutex

try_acquire used .expect on lock(), so a single panic while holding either
limiter mutex would poison it and make every subsequent auth request re-panic
— one blip became a persistent auth outage. Recover the guard with
unwrap_or_else(|e| e.into_inner()) so the limiter keeps serving.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.124.19"
+version = "0.124.20"
 edition = "2021"
 default-run = "mangalord"