device: upsert on register to enforce one record per vault+party

[johnnyluo]

Summary

• Replace unconditional db.Create() in RegisterDevice with FirstOrInit + Save so re-registering the same (vault_id, party_name) updates the existing row instead of creating a duplicate
• Add a composite uniqueIndex:idx_vault_party on (vault_id, party_name) in DeviceDBModel — AutoMigrate will create the DB-level constraint on next startup
• Switch UnregisterDevice to Unscoped().Delete() (hard delete) to prevent soft-deleted rows from blocking the unique index when a device re-registers
• Add isDuplicateKeyError helper (MySQL error 1062) to gracefully handle concurrent registration races

Test plan

• Register the same vault_id + party_name twice with different tokens — confirm only one row exists and the token is updated
• Register, unregister, then re-register the same vault_id + party_name — confirm re-registration succeeds with no duplicate key error
• Verify AutoMigrate creates idx_vault_party unique index on a fresh DB
• If running against an existing DB with duplicates, deduplicate rows before deploying (keep highest id per group)

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