feat: add Cache TTL Override per account + bump VERSION to 0.1.83

- Account-level cache TTL override: rewrite Anthropic cache_creation
  token classification (5m↔1h) in streaming/non-streaming responses
- New DB field cache_ttl_overridden in usage_log for billing tracking
- Migration 055_add_cache_ttl_overridden
- Frontend: CacheTTL override toggle in account create/edit modals
- Ent schema regenerated for new usage_log fields

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/ent/schema/usage_log.go

@@ -119,6 +119,10 @@ func (UsageLog) Fields() []ent.Field {
 			Optional().
 			Nillable(),
 
+		// Cache TTL Override 标记（管理员强制替换了缓存 TTL 计费）
+		field.Bool("cache_ttl_overridden").
+			Default(false),
+
 		// 时间戳（只有 created_at，日志不可修改）
 		field.Time("created_at").
 			Default(time.Now).