feat: metrics dashboards with SVG charts, CSV export, leaderboard (phase 10)

- aggregations over the immutable bountyAwards ledger: totals, weekly
  buckets ($dateTrunc), per-developer and per-customer groupings
- timeline-derived: approval rate, time logged, assigned→approved lead
  time, imported→published atomization lead time, open board depth
- developer + consultant dashboards (admin = global consultant view),
  date-range filters, CSV export of the ledger
- leaderboard (top 10 by bounty) honoring the new leaderboardOptOut
  profile setting
- hand-rolled SVG line and bar charts (~150 lines, no chart library)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/http/web.go

@@ -220,16 +220,7 @@ func (s *Server) routesWeb(mux *http.ServeMux) {
 	}))
 
 	rolePage("/messages", "messages.html", "Messages", "messages", "/static/js/messages.js", nil)
-
-	// Placeholders for areas built in later phases; replaced as they land.
-	placeholders := map[string]string{
-		"/metrics": "Metrics",
-	}
-	for path, title := range placeholders {
-		mux.HandleFunc("GET "+path, s.page(func(w http.ResponseWriter, r *http.Request, u *domain.User) {
-			s.render(w, r, "placeholder.html", &pageData{Title: title, User: u})
-		}))
-	}
+	rolePage("/metrics", "metrics.html", "Metrics", "metrics", "/static/js/metrics.js", nil)
 }
 
 func loginErrorMessage(code string) string {