Export daily aggregates as CSV or JSON

Returns a downloadable file (CSV uses RFC-4180 quoting; JSON is a flat array — no envelope). The format query selects the content type. When projectId is set, every row carries that project's name in the project_name column. Supply startDate + endDate (YYYY-MM-DD, together, max 366 days) for a custom period; the filename then reflects those bounds instead of the preset.