Implement `SwapPayment` model

[codebestia]

Represents a swap-routed payment where the payer pays in one token and the merchant settles in another. Specific to Shade's cross-asset capabilities.

Proposed Steps:

• Create src/shade/models/swap.py.
• Fields: id, pay_in_token, settle_out_token, amount_in: Decimal, amount_out: Optional[Decimal], routing_path: list[str], slippage_tolerance: float, status, stellar_tx_hash: Optional[str], created_at.
• Add SwapStatus enum: pending, swapping, completed, failed, slippage_exceeded.
• Validate 0 < slippage_tolerance < 1.

Acceptance Criteria:

• SwapPayment.from_dict(...) populates all fields including routing path list.
• slippage_tolerance outside (0, 1) raises ValueError.
• status is a SwapStatus enum.

[GBOYEE]

Greetings, I can handle this one. I'll implement it to spec and add tests. I'll open a PR referencing this shortly once assigned.

[sainath5001]

Hi, I would like to work on this issue.

I have worked on similar tasks before involving payment flows, token-based transactions, and model/schema implementations with validation logic. I’m familiar with designing data models, enums, serialization/deserialization (from_dict patterns), and handling validation constraints.

I can implement the SwapPayment model, SwapStatus enum, and required validations according to the acceptance criteria.

[ALIPHATICHYD]

Hi maintainer
I'd love to work on this issue
I have defi experience on this aspect

[grantfox-oss]

🦊 GrantFox — @ALIPHATICHYD 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 #41)
2. Your PR will be reviewed by the ShadeProtocol maintainers

Good luck! Track your progress on GrantFox.