V2-6: Post-Deployment Rollback

[ShortCipher5]

Extend deployment history (B7) with one-click rollback to any previously successful version, without a rebuild.

Depends on: #15 (B7: Deployment History), #12 (B4: Deployment Execution Pipeline)

References: PRD §3.8

Acceptance Criteria

• "Restore to this version" button on every previous successful deploy in history
• Rollback performs rolling swap directly from registry — no rebuild required
• Rollback recorded in history as type:rollback
• If image deleted from registry: rollback option disabled with clear explanation
• Rollback uses same 4-stage pipeline UI (Prep / Build / Launch / Live)

Notes

This was descoped from the mid-May MVP (B7) to keep rollback execution out of scope until the Deployment Execution Pipeline (B4) is stable.