feat(credit): implement subscription credit system and account balance (#719)

[Yinklekay]

Pull Request: Implement subscription credit system and account balance (Closes #719)

Summary

Adds a complete, end-to-end subscription credit system that mirrors the existing subtrackr-credit Soroban contract shape and the mobile AccountCredit ledger store, exposing:

• Per-user credit balances with oldest-first consumption and never-negative invariants.
• Credit application to subscription payments at the invoice-close boundary, so downstream payments and dunning only ever see the net amount due.
• Immutable history / audit trail with paginated queries by time and entry kind.
• A dedicated credit analytics dashboard with projected-vs-committed outstanding liability, daily usage trend, expiry forecast, top accounts, and burn-rate / consumption latency.
• Expiration management with a per-account after_secs policy and explicit expireAccount / expireAll mutations that produce auditable ledger entries.
• Bidirectional credit transfer between accounts, capped at the sender's unexpired balance.
• Credit reporting in CSV and JSON with multi-section ledger flow + wallet transaction history.

Closes #719

Acceptance Criteria

• Credit balance per user — creditService.getBalance, getAccount; mobile + web stores unchanged.
• Credit application to subscription payments — applyCreditToCharge (integrates at invoice close).
• Credit history with audit trail — paginated getAuditTrail with time/kind filters + printReport history section.
• Credit analytics dashboard — getGlobalBreakdown, getUsageTrend, getExpiryForecast, getAccountSummary, topAccountsByBalance; mobile 4-tab CreditBalanceScreen.tsx + app/stores/analyticsStore.ts snapshot.
• Credit expiration management — setExpirationPolicy, expireAccount, expireAll, getExpiryForecast (filters past-due lots).
• Credit transfer between users — transferCredit with sender/recipient ledger entries.
• Credit reporting — printReport + exportReport(..., 'csv' | 'json').

Technical Scope

A backend/services/billing/creditTypes.ts (new)
A backend/services/billing/creditService.ts (new)
A backend/services/billing/tests/creditService.test.ts (new — 36 tests)
M backend/services/billing/interfaces.ts (added ICreditService)
M backend/services/billing/errors.ts (added credit error codes + factories)
M backend/services/billing/index.ts (exported CreditService / types / helpers)
M app/stores/creditStore.ts (fixed pre-existing compile error; behaviour-preserving rewrite)
M app/screens/CreditBalanceScreen.tsx (rebuilt: 4-tab analytics dashboard UI)
M app/stores/analyticsStore.ts (added CreditMetricSnapshot + aggregation)

Architecture

• Ledger model is source of truth, mirroring the on-chain Soroban contract and the mobile AccountCredit store. A future contract-event replay integration is translation-free.
• Read methods are strictly non-mutating via projectExpiry(). Mutating expiry surface is explicit (expireAccount / expireAll).
• Downstream payments and dunning only see the net amount due via applyCreditToCharge.
• Projected vs committed outstanding liability are both exposed so finance dashboards can show on-chain lag.

Test Plan

• Backend Jest: 36/36 tests pass (npx jest -c jest.backend.config.js backend/services/billing/__tests__/creditService.test.ts).
• Root tsc --noEmit: clean.
• Backend tsc --noEmit -p tsconfig.json: clean for the credit files.
• eslint --no-ignore on modified files: 0 errors (4 Array style warnings).

Risks / Notes

• No persistence layer is added in this PR. The ICreditService interface makes a DB-backed repository a drop-in replacement.
• Past-due amounts are surfaced as a single delta in the LAST bucket of getUsageTrend (a "now" signal). A future per-day historical expiry trend would be a separate projectExpiryTrend helper.
• The on-chain Soroban contract (contracts/credit/src/lib.rs) already implements the same operation set and is not modified by this PR.

Reviewer Checklist

• app/stores/creditStore.ts rewrite preserves the 8 existing tests.
• creditService is class+instance with ICreditService re-exported.
• All mutators throw typed BillingError with appropriate codes.
• Read methods are strictly non-mutating.
• CreditBucketBreakdown returns both projected and committed outstanding liability.
• CSV report correctly escapes commas.
• No new lint errors introduced.

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[Smartdevs17]

merge conflicts

[Yinklekay]

done