Task: Implement config-driven super-admin bootstrap in Go

Identity

Add config-driven first-super-administrator bootstrap to the Go backend, replacing the previous scripts/bootstrap-admin.mjs convention with startup-time creation from environment configuration.
Human direction (2026-08-14): bootstrap credentials come from configuration, not a script or CLI step; there is no legacy account import requirement; production is a first deployment of the Go stack, not a cutover from a live Next.js deployment.

Bootstrap only when the backend is PostgreSQL; local development keeps the seeded demo administrator.
Create exactly once: skip when any super administrator exists, including disabled ones.
Fail application startup on a misconfigured bootstrap (invalid phone, password shorter than 8 characters, database error) so the problem is visible instead of silently missing.
Reuse the existing administration.Service validation, role model, and password hashing; no new store surface.

Added backend/internal/application/bootstrap_admin.go:
- BootstrapAdminConfig with Configured().
- ParseBootstrapAdminConfig(getenv) reading ZHINIAN_BOOTSTRAP_ADMIN_PHONE, ZHINIAN_BOOTSTRAP_ADMIN_PASSWORD, ZHINIAN_BOOTSTRAP_ADMIN_NAME (default 平台超级管理员), mirroring the previous script variable names.
- BootstrapSuperAdmin(ctx, backend, service, config) performing the idempotent creation.
Wired into application.New right after the administration service is composed; a successful bootstrap logs one line, a failure aborts startup with bootstrap super administrator: ....
Added backend/internal/application/bootstrap_admin_test.go with an in-memory fake store covering: config parsing/defaults, no-op without config, no-op on local backend, first-bootstrap creation with hashed password, second-run idempotency, skip when a disabled super admin exists, and short-password rejection.
Documented the three variables and the startup behavior in backend/README.md.

CGO_ENABLED=0 go test -count=1 ./... — all 19 packages PASS (the local plain go test crashes with a macOS dyld missing LC_UUID load command issue unrelated to this change; the repository runner always uses CGO_ENABLED=0).
CGO_ENABLED=0 go vet ./... — PASS.
CGO_ENABLED=0 go build ./cmd/zhinian-api — PASS.
CGO_ENABLED=0 go test -race -count=1 ./internal/application/ -run Bootstrap — PASS.

Production schema initialization now happens by manually executed SQL instead of the ACK migration Job pod; see the 2026-08-14 human direction. Deployment guidance and canonical memory reconciliation for that change are tracked in the follow-up integration task.