Add Core Issue Loop seed data and regression checks

[mrboxs]

This was generated by AI during triage.

What to build

Add the minimal seed data and regression checks needed to verify the Core Issue Loop end-to-end. This should make local development reproducible without pulling in deferred V1 features such as Private Teams, Projects, Cycles, Labels, Comments, Attachments, Command Menu, or Workspace Search.

The result should give a human reviewer a reliable demo path and a test suite that catches regressions in OAuth return behavior, Workspace Creation, Welcome Flow, Issue creation, Issue reading, Issue updates, filtering, redirects, and access control.

Acceptance criteria

• A minimal seed creates one Workspace, one public Team, default Workflow Statuses, a Team Issue Counter, and several Issues across statuses.
• Seed data does not create Private Teams or deferred Core Issue Loop non-goals.
• Local setup documentation or scripts reflect the pre-V1 drizzle-kit push workflow.
• Tests cover protected oRPC procedure rejection when unauthenticated.
• Tests cover OAuth callback/new user landing on /.
• Tests cover signed-in users with no Workspace seeing Workspace Creation on /.
• Tests cover Workspace Creation creating Workspace, Default Team, Team key, default Workflow Statuses, Team Issue Counter, active session Workspace, and active session Team.
• Tests cover Workspace Creation redirecting to /:workspaceSlug/welcome.
• Tests cover Welcome Flow steps being completed or skipped according to scope.
• Tests cover Slack Welcome step being skip-only and requiring no Slack credentials.
• Tests cover Welcome finish redirecting to /:workspaceSlug/team/:teamKey/active.
• Tests cover old plural Team routes redirecting to singular canonical routes.
• Tests cover cross-workspace access to /:workspaceSlug/welcome and Team pages returning authorization-safe errors.
• Tests cover sequential Issue key generation, including a concurrent request scenario.
• Tests cover Issue list and Issue detail cross-workspace denial.
• Tests cover Issue update rejection for statuses from another Team and assignees outside the Workspace.
• Tests cover Completed and Canceled timestamp transitions.
• bun run lint, bun run fmt:check, bun run test, and bun --bun run build pass.

Blocked by

• Harden Core Issue Loop access control #49

[linear-code]

BOXS-51