Use exact decimal arithmetic for payments, fees, quotes, and limits

[nwosum113-ui]

Summary

Switch from floating-point to decimal arithmetic for all payment-related calculations to ensure precision and avoid rounding errors.

Changes

• Replaced floating-point arithmetic with decimal-based operations in payment, pricing, and compliance modules
• Updated Prisma schema and migration files to support decimal fields for monetary values
• Modified validation logic to handle decimal strings and prevent precision loss
• Added unit tests for decimal arithmetic operations
• Updated Stellar wallet adapter to work with decimal amounts

Impact

• Payment Orchestrator: Now uses exact decimal calculations for all transactions
• Pricing Service: Ensures accurate rate calculations without floating-point inaccuracies
• Compliance Service: Maintains precise financial records for auditing
• Wallet Adapter: Handles decimal amounts correctly in Stellar transactions
• Database Schema: Decimal fields replace float types for monetary columns

Closes #154

Remaining hardening is tracked separately in #181.

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
send-am-admin	Skipped		Aug 20, 2026 2:06pm
senda	Skipped		Aug 20, 2026 2:06pm

[Gozirimdev]

This PR cannot be merged as linked. It says it closes #160, but #160 is about voice-note security and privacy; this PR contains decimal-payment changes.

Please either:

• change the PR to implement issue #160 fully, or
• link the correct issue and remove the incorrect closure.

If this is intended as decimal work, also fix mixed-asset compliance totals, exact rate parsing, configured fiat support, and existing-value reconciliation. It also overlaps with PR #171, so the duplicate work needs to be resolved.

[Gozirimdev]

The core decimal-arithmetic implementation is substantial and the remaining precision-policy, currency-separation, and reconciliation work is tracked in unassigned issue #181.