Replace Spring Retry CircuitBreakerRetryPolicy with lightweight custom
implementation using ConcurrentHashMap and AtomicReference<Instant>.
Avoids classpath conflict between spring-retry and spring-aspects
RetryTemplate, and eliminates stacked retries with Feign @Retryable.
Behavior: circuit opens for 30s after first failure, returns
fallback immediately without hitting external APIs. Transitions
through CLOSED -> OPEN -> HALF_OPEN -> CLOSED automatically.
Applied to AniListService and MyAnimeListService for all
searchMangaByTitle, getMangasByTitle, and getMangaDataById calls.
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