[ENHANCE] Add `address` field to `Account` model, & other multi-chain updates (not urgent)

[lachlanglen]

Summary

As we move to support multi-chain, we will need to make two adjustments to the Account model.

These are not urgent, especially since NEAR and Stellar addresses are not compatible, but should be done before multiple EVM chains are supported.

1. Store account address in Account.address instead of Account.id PK (this is because the same address may exist on multiple chains
2. Use AutoField for Account.id PK instead of CharField
3. Add unique_together constraint for Account.chain & Account.address to ensure that only one Account record exists per address per chain

This will require careful review and local testing before deploying & migrating dev/prod data.

Motivation

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Description

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Alternatives

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Risks

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Acceptance Criteria

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Additional Information

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