Get Signup Activity

Per-day MCP tool-call activity for every user who signed up on/after start_date, for the signups dot plot. One object per new user with a calls_by_day map, the total, and the day they first connected via an MCP client — all bucketed into calendar days of tz (the launch is reasoned about in Singapore time). Aggregated in SQL so heavy users are counted in full rather than truncated to a page of recent rows, and so the whole grid is one query instead of one request per user.