fix(security): make email storage/representation consistent across register, profile, and lookup

[cypriannwokolo2-creator]

Summary

Registration stores plaintext email (auth_handler.go:308), profile update stores hashUserEmail(email) (user/service.go:188), while FindByEmail always hashes using another "EMAIL:"+sha256[:16] scheme in auth_handler.go:722. Registered emails never match FindByEmail - a data-integrity and PII-consistency bug.

Acceptance Criteria

• Single canonical email representation (decide hash vs plaintext + encryption)
• All writes and lookups use the same transform
• Data-migration/backfill for existing rows
• Tests for round-trip consistency

Complexity

High.

[Paulotoide]

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[mondaysaidu18-lgtm]

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