feat: configurable minimum deposit size to prevent dust LP positions

[abayomicornelius]

Summary

Very small deposits (e.g., 0.01 USD) create market token entries that cost more in rent than their pool contribution. A minimum deposit size prevents dust accumulation and simplifies keeper economics.

Proposed Design

Store min_deposit_usd_key(market) in data_store (admin-configurable, default 0 = no minimum).

In exchange_router::create_deposit, compute the USD value of the deposited tokens and revert with DepositError::BelowMinimumDeposit if below the minimum.

Acceptance Criteria

• Deposit below minimum reverts before vault transfer (no tokens moved)
• Minimum of 0 allows any deposit size (no regression)
• USD value computed using current oracle price at creation time
• Integration test: min deposit = 100 USD, deposit 50 USD — revert
• Integration test: deposit exactly 100 USD — succeeds

Affected Contracts

exchange_router, data_store

[tobiadewola41-eng]

@tobiadewola41-eng has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue because

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