Migrate `BalanceLedger.amount` from Float to Decimal with running totals

[kilodesodiq-arch]

Problem Statement. BalanceLedger.amount is Float. It's the canonical source of
locked-fund totals; corruption in this column breaks the entire claim workflow.

Technical Context. #29 also touches this.

Expected Outcome. Schema migration + a denormalised running total column
(BalanceLedgerRunning.totalLocked, last 24h snapshot) computed by a scheduled job.

Acceptance Criteria.

• An end-to-end test asserts that the running total matches
  Σ BalanceLedger.amount for the last 24 h.

Files or modules likely to be affected.
prisma/schema.prisma,
prisma/migrations/*,
new src/jobs/balance-ledger-snapshot.job.ts.

Dependencies. #29.

Difficulty. Medium
Estimated effort. M

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Backlog item #53 from `docs/maintainer-issue-backlog.md.

[DevSolex]

Hi Maintainer,

I’m a smart contract and full-stack developer and would love to contribute to this issue. I’m an experienced contributor with hands-on expertise in Rust, Soroban, Cairo, and TypeScript.

I’m passionate about building secure, scalable blockchain solutions, with a strong emphasis on writing clean and maintainable code. Based on the scope of this issue, I’m confident I can deliver a complete solution within 24 hours of assignment.

I’d greatly appreciate the opportunity to contribute and help move this forward.

[grantfox-oss]

🦊 GrantFox — @DevSolex 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 #261)
2. Your PR will be reviewed by the ChainForgee maintainers

Good luck! Track your progress on GrantFox.