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- subdivide (async 202, atomizing status, re-run replaces unpublished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

@@ -33,8 +33,13 @@ func EnsureIndexes(ctx context.Context, db *mongo.Database) error {
 			{Keys: bson.D{{Key: "assignee.userId", Value: 1}, {Key: "status", Value: 1}}},
 			{Keys: bson.D{{Key: "rootId", Value: 1}}},
 			{Keys: bson.D{{Key: "parentId", Value: 1}}},
+			// The spec calls this "unique sparse", but a compound sparse index
+			// matches any doc where customerId exists (i.e. every task), which
+			// breaks subdivided children. A partial index expresses the intent:
+			// uniqueness only for tasks that actually have an external ref.
 			{Keys: bson.D{{Key: "external.system", Value: 1}, {Key: "external.key", Value: 1}, {Key: "customerId", Value: 1}},
-				Options: options.Index().SetUnique(true).SetSparse(true)},
+				Options: options.Index().SetUnique(true).SetName("uniq_external_ref").
+					SetPartialFilterExpression(bson.M{"external.key": bson.M{"$exists": true}})},
 			{Keys: bson.D{{Key: "title", Value: "text"}, {Key: "description", Value: "text"}}},
 		},
 		"bountyAwards": {
@@ -63,6 +68,10 @@ func EnsureIndexes(ctx context.Context, db *mongo.Database) error {
 		},
 	}
 
+	// migration: an earlier build created the external-ref index as sparse
+	// under the auto-generated name; drop it so the partial variant applies
+	_ = db.Collection("tasks").Indexes().DropOne(ctx, "external.system_1_external.key_1_customerId_1")
+
 	for coll, models := range plan {
 		if _, err := db.Collection(coll).Indexes().CreateMany(ctx, models); err != nil {
 			return fmt.Errorf("create indexes for %s: %w", coll, err)

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)
 					}
 				}
 			}