Summary

• Advances [Release] Verify backend account and data deletion behavior #439 and references parent track [Release Track] Privacy, account deletion, and Data safety #464.
• Adds a decision-complete plan for backend account/data deletion verification.
• Adds a backend-owner handoff checklist with provider evidence, data deletion/retention matrix, and privacy policy cross-check tasks.

Verification

• git diff --cached --check - passed before commit.
• No Flutter tests run; documentation-only change.

Remaining Human Tasks

• Backend owner must complete provider verification for normal, Google, and Apple release auth flows.
• Backend owner must document deletion/retention behavior for each data category.
• Privacy/product owner must reconcile verified backend behavior with the privacy policy draft before [Release] Verify backend account and data deletion behavior #439 can close.

Status

This PR advances #439 but does not close it because server-side deletion truth requires backend/environment access.