fix: folio reversal loop + payment refund UI (closes #166)

[telivity-otaip]

Summary

Fixes #166 (contributor Gugabit): folio charge Reverse could be clicked repeatedly (including on reversal rows), creating a negative-balance deadlock, and captured payments had no refund action in the dashboard.

Note: supersedes #169 — that branch became push-locked after the first commit; this branch includes the same fix plus the README/docs/test-stats.json sync that CI requires after adding 2 tests.

Root causes

1. Backend hole: reverseCharge blocked a second reverse of the same original, but never rejected reversing a row that itself had isReversal: true. Reversing reversal R created R2 = -R, then R3, indefinitely. The same hole existed on the direct postCharge path when posting isReversal: true against a reversal row.

2. UI gating bug: Folios used a phantom c.isReversed field that is not in the schema/API (only isReversal + originalChargeId exist), so Reverse showed on every unlocked open-folio row.

3. Missing refund UI: POST /v1/payments/:id/refund already existed with full/partial refund support. The Folios payments list simply rendered no action button. Negative direct payments stay rejected by design (CreatePaymentDto positive-only); refunds are the sanctioned path.

Changes

API

• Reject reversing a reversal in reverseCharge and in postCharge when isReversal targets another reversal (Cannot reverse a reversal transaction).
• Operational-integrity rule (not a KB domain invention): undo a mistaken reversal by re-posting the charge.

Dashboard

• Derive reversed originals from isReversal + originalChargeId; hide Reverse on reversal rows and already-reversed charges; strikethrough originals that have been reversed.
• Add Refund on open-folio payments in captured / settled / partially_refunded (excludes negative refund-child rows).
• Toast error feedback on reverse/refund failures.

Tests / docs

• Unit tests for reverse-of-reversal rejection on both service paths (+2 tests).
• Synced README + docs/test-stats.json to 1153 tests (fixes CI “README test counts are stale”).
• folios.refund (+ confirm/success/failure keys) in en / pt-BR; minimal folios keys in de.

Verification

• Folio + payment unit suites green locally
• pnpm readme:sync-tests → 1153 / 139

Closes #166