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- subdivide (async 202, atomizing status, re-run replaces unpublished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/store/store_integration_test.go

@@ -28,7 +28,7 @@ func TestEnsureIndexesIsIdempotent(t *testing.T) {
 	// Spot-check a few critical indexes actually exist.
 	checks := map[string]string{
 		"users":    "email_1",
-		"tasks":    "external.system_1_external.key_1_customerId_1",
+		"tasks":    "uniq_external_ref",
 		"sessions": "expiresAt_1",
 	}
 	for coll, wantName := range checks {
@@ -44,10 +44,10 @@ func TestEnsureIndexesIsIdempotent(t *testing.T) {
 		for _, s := range specs {
 			if s["name"] == wantName {
 				found = true
-				// the unique upsert key must really be unique+sparse
+				// the upsert key must be unique and partial (external-only)
 				if coll == "tasks" {
-					if s["unique"] != true || s["sparse"] != true {
-						t.Errorf("%s/%s: want unique sparse, got %v", coll, wantName, s)
+					if s["unique"] != true || s["partialFilterExpression"] == nil {
+						t.Errorf("%s/%s: want unique partial, got %v", coll, wantName, s)
 					}
 				}
 			}