feat: atomization pipeline with job queue, atomizer client, WS hub, and board UI (phase 7)

- internal/extsvc: circuit breaker (§11.16) + retrying bearer JSON client
- atomizer client: §5.1 contract incl. defensive coefficient normalization
  and extension coefficient range (0,2]
- persisted jobs collection: atomic claim, panic recovery, exponential
  backoff (max 3), stale-running requeue, shutdown-safe bookkeeping
- subdivide (async 202, atomizing status, re-run replaces unpublished
  children after confirm) and extend (sibling task, source budget)
- failure path reverts status and notifies the consultant on final attempt
- PATCH task editing with bounty recompute, budget cascade to descendants
- publish single + bulk; task detail endpoint role-scoped
- coder/websocket hub: multiplexed channels, origin check, 30s heartbeats;
  client ws.js with reconnect + 15s polling fallback
- consultant atomization board: tree by root, coefficient sliders with live
  per-parent sum indicator, bounty preview, modals, shimmer, atomizer-down
  gating via /api/v1/service-health
- fix: tasks external-ref unique index is partial, not sparse (compound
  sparse indexed every task through customerId and broke child inserts)

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

internal/http/web.go

@@ -181,11 +181,21 @@ func (s *Server) routesWeb(mux *http.ServeMux) {
 		})
 	}))
 
+	mux.HandleFunc("GET /consultant/board", s.page(func(w http.ResponseWriter, r *http.Request, u *domain.User) {
+		if !u.Roles.Consultant && !u.Roles.Admin {
+			s.render(w, r, "placeholder.html", &pageData{Title: "Not authorized", User: u})
+			return
+		}
+		s.render(w, r, "atomization.html", &pageData{
+			Title: "Atomization Board", Active: "atomization", User: u,
+			Scripts: []string{"/static/js/atomization.js"},
+		})
+	}))
+
 	// Placeholders for areas built in later phases; replaced as they land.
 	placeholders := map[string]string{
 		"/board":              "Bounty Board",
 		"/my-tasks":           "My Tasks",
-		"/consultant/board":   "Atomization Board",
 		"/consultant/reviews": "Review Queue",
 		"/consultant/pool":    "Developer Pool",
 		"/messages":           "Messages",