activity graph on owner pages #30

A GitHub-style contribution/activity graph on user and org pages. hutch-stats had to reconstruct this externally for sourcehut; it is far easier when the platform records an activity signal at event time. The events table already records issue/MR/repo events with actor and timestamp — extend coverage to commits (post-receive: author-attributed, dedup by sha) and comments, then aggregate per user per day for the graph. Render server-side (SVG or CSS grid), no JS required.