Add the credits over time panel to the DAP Impact Spend view

What does this MR do and why?

Adds the "Credits over time" panel to the DAP Impact Spend view: stacked credits per flow type on the DuoWorkflows engine, bucketed daily or weekly to match the selected range. Same shape as the Work tab's Sessions over time, so every flow type appears, chat included. The prototype excluded chat on an assumed credit share that production data cannot confirm yet, so we ship the simple chart and revisit if real data shows chat drowning the rest.

Charts render the first 100 rows until !256013 (merged) merges, which this MR depends on. Behind the default-off dap_impact_v1 flag. Validated in GDK against seeded credits data.

References

https://gitlab.com/groups/gitlab-org/-/work_items/23470

Screenshots or screen recordings

From a GDK with seeded duo_workflows_workflows_enriched data:

How to set up and validate locally

Enable the dap_impact_v1 feature flag, seed duo_workflows_workflows_enriched with credits data, then open /explore/analytics_dashboards/dap_impact?view=3.