Feat/issues 626 629 637 639

[pope-h]

Summary

• optimize Ledger Monitor polling by narrowing the hot query, loading merchant notification config lazily, and adding a partial index for pending-payment scans
• tighten Transaction Signer rate limiting with a unified verify-payment limiter keyed per payment/requestor and aligned polling defaults
• improve Portfolio Chart Widget screen reader support with an accessible region, live summary, and hidden data table alternative
• refactor Transaction Filter Sidebar state into shared reducer/helpers and debounce search-driven URL updates

Testing

• cd backend && npx vitest run src/lib/horizon-poller.test.js src/lib/rate-limit.test.js
• frontend static checks via Problems API for:
  • frontend/src/components/PaymentMetrics.tsx
  • frontend/src/components/PaymentMetrics.test.tsx
  • frontend/src/lib/payment-history-filters.ts
  • frontend/src/lib/payment-history-filters.test.ts
  • frontend/src/app/(authenticated)/payment-history/page.tsx

Notes

• frontend vitest is not installed in the current environment, so frontend validation here is static rather than executable
• left the existing unrelated local changes in frontend/src/components/KycSubmissionForm.tsx untouched

Closes #626
Closes #629
Closes #637
Closes #639

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