GroupConfig gains CacheTTL (json cacheTTL, empty inherits the global
cache.ttl). Parsed and validated at construction (newAuthStore,
groupCacheTTL in cache.go): a malformed duration, a duration <= 0, or
CacheTTL set while the global cache block is not configured is a
constructor error naming the group, mirroring Cache:true's existing
"nothing to inherit from" check.
group carries the resolved time.Duration (0 means inherit).
responseCache.store gains an explicit ttl parameter; routes_unified.go
computes effectiveTTL(g.cache, grp) immediately before each store call,
so setEx's Redis-native EX seconds reflect the group's own override.
Tests: constructor-error table, empty-inherits and valid-override
resolution, effectiveTTL inheritance/override, a scripted-RESP-server
proof that store's SET...EX carries a different value per group, and an
end-to-end Gateway.ServeHTTP test with two groups asserting the actual
wire EX value (30 vs 60) differs per group.
README: GroupConfig.cacheTTL config row and a Caching-section paragraph.
lukaszraczylo