core: add contract storage migration utilities

[EmeditWeb]

Problem

When contracts are upgraded, new storage fields are
incompatible with existing stored data. There is no
migration system.

What To Build

1. Add schema_version to each contract
2. Add migrate() function to each contract
3. Call migrate() automatically in upgrade()

Files To Touch

• contracts/creditline-contract/src/lib.rs
• contracts/creditline-contract/src/storage.rs
• All contract lib.rs files

Acceptance Criteria

• schema_version stored in all contracts
• migrate() runs on upgrade
• Existing data preserved through migration
• Migration history prevents double-run

Mandatory Checks Before PR

• cargo build passes
• All tests pass
• PR references this issue

[Iwayemi-Kehinde]

I would like to work on this issue as I have reviewed the scope, I should have a PR Ready in 50 minutes.

[grantfox-oss]

🦊 GrantFox — @Iwayemi-Kehinde has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #24)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.