fix: Redis 缓存构造修正 + TTL=0 永不过期支持

- main.py: 先创建 aioredis 客户端再传入 RedisResponseCache
- redis_cache.py: ttl_s=None 时不设过期时间
- .env: REDIS_CACHE_TTL=0（永不过期）

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.py

@@ -85,9 +85,12 @@ def _build_adapters(settings: InfraSettings, embed_cfg: dict) -> _Adapters:
     cache = None
     if settings.redis_url:
         try:
+            import redis.asyncio as aioredis
             from adapters.redis_cache import RedisResponseCache
 
-            cache = RedisResponseCache(redis_url=settings.redis_url, ttl=settings.redis_cache_ttl)
+            redis_client = aioredis.from_url(settings.redis_url, decode_responses=True)
+            ttl_s = settings.redis_cache_ttl if settings.redis_cache_ttl > 0 else None
+            cache = RedisResponseCache(redis=redis_client, ttl_s=ttl_s)
         except Exception:
             logger.warning("Redis 缓存不可用，降级为无缓存模式")