* feat: add keyset u32 mapping migration and duplicate handling
- Add new database migration (version 3) to include u32 representation for keysets
- Implement migration for both redb and SQL databases
- Add duplicate detection and handling for keyset entries
- Create unique index constraint for keyset_u32 column in SQL
- Update keyset storage to include u32 identifiers
- Handle backwards compatibility for existing databases
* chore: clippy
* refactor(cashu): simplify keyset ID verification logic
- Consolidate match expression into a single expression
- Use direct comparison with ensure_cdk macro
- Improve readability of keyset ID validation
* refactor(cdk): rename `fetch_keyset_keys` to `load_keyset_keys` for clarity
- Renamed `fetch_keyset_keys` to `load_keyset_keys` across multiple modules to better reflect its behavior of loading keys from local storage or fetching from mint when missing.
- Added debug logging to indicate when keys are being fetched from the mint.
- Simplified key loading logic in `update_mint_keysets` by removing redundant existence checks.
* chore: remove unused vec
The primary purpose of this new crate is to have a common and shared codebase
for all SQL storage systems. It would force us to write standard SQL using best
practices for all databases.
This crate has been extracted from #878
