feat: reset admin credentials and enforce PR size limit

[MengMengCode]

Summary

• allow the root-only CLI management menu to reset the administrator username and password without requiring the previous password
• keep the current username when the username prompt is left empty
• revoke existing Web sessions after credentials change
• add a 5,000 changed-line pull request limit that automatically closes oversized PRs

Validation

• go test ./cmd/... ./internal/... ./web -count=1 -timeout 180s
• credential reset tests cover username/password replacement, old credential rejection, session revocation, and invalid input
• PR limit shell script passes Bash syntax validation

[Copilot]

Pull request overview

This PR adds a root-only recovery flow to reset the administrator username/password (without needing the previous password), revokes existing sessions when credentials change, and introduces a GitHub Actions workflow to automatically close pull requests exceeding a 5,000 changed-line limit.

Changes:

• Add ResetAdminCredentials to the auth service with input validation and session revocation on credential changes.
• Update the root management CLI menu to reset admin credentials and keep the existing username when the prompt is left empty.
• Add a pull_request_target workflow that enforces a 5,000 changed-line PR limit by closing oversized PRs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/auth/service.go	Adds ResetAdminCredentials recovery API layered on EnsureAdmin.
internal/auth/service_test.go	Adds tests for reset behavior, invalid input, and session revocation.
cmd/vocat/menu.go	Replaces “change password” flow with a “reset credentials” flow and new username prompt behavior.
cmd/vocat/menu_test.go	Adds a prompt regression test ensuring no “current password” prompt is shown.
.github/workflows/pr-size-limit.yml	Adds an Actions workflow to close PRs exceeding 5,000 changed lines and comment on the PR.

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