feat: add key staking rewards with 30-day lock and pro-rata reward claims

[temieehade-coder]

Closes #786

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[Chucks1093]

❌ CI Failed — verify (Contracts CI)

The verify check is failing on this PR.

Likely causes:

• New function or storage key not following the Soroban TTL bump pattern
• Missing #[contracttype] or #[contracterror] derive on a new type
• Formatting not applied before push

Steps to fix:

1. Run cargo build locally and fix any errors
2. Run cargo fmt --all and commit the result
3. Push — CI will re-run automatically