aljaz/cdk

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# Implement Saga Pattern for Swap Operations with Recovery Mechanism

## Overview

This PR refactors the swap operation implementation to use the saga pattern - a distributed transaction pattern that provides reliable transaction management through explicit state tracking and compensation-based error handling. The implementation includes a robust recovery mechanism that automatically handles swap operations interrupted by crashes, power loss, or network failures.

## What Changed

**Saga Pattern Implementation:**
- Introduced a strict linear state machine for swaps: `Initial` → `SetupComplete` → `Signed` → `Completed`
- New modular `swap_saga` module with state validation, compensation logic, and saga orchestration
- Automatic rollback of database changes on failure, ensuring atomic swap operations
- Replaced previous swap implementation (`swap.rs`, `blinded_message_writer.rs`) with saga-based approach

**Recovery Mechanism:**
- Added `operation_id` and `operation_kind` columns to database schema for tracking which operation proofs belong to
- New `recover_from_bad_swaps()` method that runs on mint startup to handle incomplete swaps
- For proofs left in `PENDING` state from swap operations:
  - If blind signatures exist: marks proofs as `SPENT` (swap completed but not finalized)
  - If no blind signatures exist: removes proofs from database (swap failed partway through)
- Database migrations included for both PostgreSQL and SQLite

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src

Swap saga (#1183 )

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tests

Introduce cdk-sql-common

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build.rs

fix(build): use OUT_DIR for migration files (#1088 )

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Cargo.toml

Swap saga (#1183 )

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README.md

Introduce cdk-sql-common

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README.md

CDK SQL Base

This is a private crate offering a common framework to interact with SQL databases.

This crate uses standard SQL, a generic migration framework a traits to implement blocking or non-blocking clients.

ALPHA This library is in early development, the API will change and should be used with caution.

Features

The following crate feature flags are available:

Feature	Default	Description
`wallet`	Yes	Enable cashu wallet features
`mint`	Yes	Enable cashu mint wallet features
`auth`	Yes	Enable cashu mint auth features

License

This project is licensed under the MIT License.