Implement `Payment` model

[codebestia]

Represents a Shade payment resource returned by the API. All monetary amounts should be stored as Decimal to avoid floating-point precision issues.

Proposed Steps:

• Create src/shade/models/payment.py.
• Fields: id: str, status: str, amount: Decimal, currency: str, description: Optional[str], merchant_id: str, stellar_tx_hash: Optional[str], payment_address: str, created_at: datetime, updated_at: datetime.
• Add a PaymentStatus StrEnum with values: pending, completed, cancelled, expired, partially_paid.
• Validate that amount > 0.

Acceptance Criteria:

• Payment.from_dict(api_response) constructs the model without error.
• payment.amount is a Decimal, not a float.
• payment.status is a PaymentStatus enum value.
• Invalid status strings raise a clear validation error.

[sainath5001]

Hi, I would like to work on this issue.

I have experience implementing blockchain-related data models, payment flows, and validation logic. I’m familiar with designing strongly typed models, handling API response mappings, using Decimal for financial precision, and implementing enums with validation.

I can implement the Payment model, PaymentStatus enum, amount validation, and ensure all acceptance criteria are covered.

[grantfox-oss]

🦊 GrantFox — @sainath5001 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #37)
2. Your PR will be reviewed by the ShadeProtocol maintainers

Good luck! Track your progress on GrantFox.