docs(plan): fix unclosed code fence in Task 2, bump api build image to golang:1.26

docs/superpowers/plans/2026-09-04-backend.md

@@ -604,7 +604,6 @@ func (s *Store) CountAdmins(ctx context.Context) (int, error) {
 	return n, err
 }
 
-```go
 // ---------- libraries ----------
 
 func (s *Store) CreateLibrary(ctx context.Context, name, root string) (int64, error) {
@@ -4212,7 +4211,7 @@ git add backend && git commit -m "feat(backend): main wiring, graceful shutdown,
 `deploy/Dockerfile.api`:
 
 ```dockerfile
-FROM golang:1.23-alpine AS build
+FROM golang:1.26-alpine AS build
 WORKDIR /src
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download