New Customer Effort Score (CES) metric

We added a new metrics endpoint, GET /stats/ces, for Customer Effort Score questions.

Each aggregated item returns:

• avg: the average effort score;
• high_effort, moderate_effort, low_effort: the number and percentage of answers in each effort band.

It supports the same filtering and grouping parameters as the other stats endpoints. The ces question type (scale from 1 to 7) is now part of the API schema.

See CES - Customer Effort Score for details.