Per-Shift coverage roll-up for a recurring signup

One entry per Shift — a repeating slot in the signup's Slot Series — rolled up across every date it runs. Answers 'which Shift is chronically short across its upcoming dates?', which the flat, event-wide /analytics snapshot cannot. Returns an empty list (200) for a signup with no Slot Series; a non-recurring signup simply has no Shifts. Regulars carry participant names and attendance counts and NO email addresses: this is an analytics roll-up, and the capability it owes is 'who are my dependable people for this Shift'. Use /signups/{id}/participants under its own scope to reach anyone. Not paginated, deliberately: the roll-up averages over every Occurrence and the roster read behind it is unpaged, because an average over a page is an average of the first 50 people. It is bounded upstream instead — a Slot Series generates at most 2000 Occurrences (MAX_OCCURRENCES in slot-series.service.ts) and each Shift publishes at most four Regulars, so the worst case is on the order of 2000 coverage cells.