[DATA INTEGRITY][WALLET SCHEMA] Migrate duplicate walletAddress fields to one canonical key

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Summary

The User model maintains both walletAddress and walletaddress as independently unique fields. Reads and writes must continually fall back between them, allowing split-brain identity and uniqueness behavior.

Repository evidence

• models/User.ts:89-99 declares both fields with separate unique sparse indexes.
• app/api/users/[id]/route.ts:236-242 reads either field and writes both to keep them synchronized.

Scope

• Select one canonical field and add a conflict-detecting migration.
• Remove fallback writes and replace both indexes with one canonical uniqueness constraint.

Acceptance criteria

• Every user has at most one canonical wallet identity and no two users collapse during migration.
• Old field access is removed after a measured compatibility window.

Tests

• Cover only-old, only-new, matching-both, conflicting-both, and cross-user duplicate records.

Non-goals

• Changing the authentication provider.

[Owolabenjade]

Hello maintainer, I want to work on this issue.

[grantfox-oss]

🦊 GrantFox — @Owolabenjade has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #192)
2. Your PR will be reviewed by the Chainmove maintainers

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