feat: PDF receipts, Stellar fee estimation, and i18n notifications

[Idaonoli]

Summary

Implements three related enhancements from the Stellar Wave milestone:

• Add transaction receipt PDF generation API endpoint #948 — GET /api/remittances/:id/receipt returns a branded PDF receipt for any remittance. Authentication is required via X-API-Key (admin) or X-Sender-Address (must match the remittance owner).
• Add Stellar network fee (XLM) estimation and top-up guidance #949 — GET /api/accounts/:address/stellar-fees returns the current Horizon base fee, the account's native XLM balance, and the estimated XLM cost for N operations. Includes a low_balance flag and top_up_link when balance is below 2 XLM. An estimateStellarFee(operationCount) helper is added to the SDK. The frontend gains a checkXlmBalance() utility that calls the endpoint and renders a red warning banner when XLM is low.
• Add multi-language support for email and SMS notifications #950 — Notification templates are extracted into per-locale files (en, es, fr, pt) covering all five event types: remittance_created, remittance_completed, remittance_failed, kyc_approved, kyc_expired. A preferred_language column is added to notification_preferences (with migration). The notification service loads the correct locale template and falls back to English when a translation is missing. A CI-friendly script (check-notification-templates.ts) exits 1 if any locale is missing any key.

Changes

Area	File(s)	Change
API	api/src/routes/remittances.ts	Added GET /:id/receipt endpoint
API	api/src/services/receiptGenerator.ts	New — PDFKit-based receipt generator
API	api/src/routes/accounts.ts	New — Stellar fee + XLM balance endpoint
API	api/src/app.ts	Registered /api/accounts router
API	api/package.json	Added pdfkit + @types/pdfkit dependencies
Backend	backend/src/notification-service.ts	i18n template lookup, preferred_language in preferences, notifyKycEvent()
Backend	backend/src/notification-templates/	New — types.ts, index.ts, en.ts, es.ts, fr.ts, pt.ts
Backend	backend/migrations/	Migration to add preferred_language column
Backend	backend/scripts/check-notification-templates.ts	New — CI completeness check
SDK	sdk/src/index.ts	Added estimateStellarFee() helper
Frontend	frontend/src/main.ts	Added checkXlmBalance() and low-XLM banner

Notes

• Run npm install in api/ after merging to pick up pdfkit.
• Run npm run migrate in backend/ to apply the preferred_language column migration.
• Add tsx backend/scripts/check-notification-templates.ts to CI to enforce template completeness.
• The PDF receipt endpoint requires ADMIN_API_KEY to be set in the environment for admin access. Sender-based auth uses the X-Sender-Address header matched against remittance.sender_id.
• STELLAR_EXPLORER_URL can be set to override the default on-chain explorer base URL in receipts.

Test plan

• GET /api/remittances/:id/receipt with a valid X-API-Key returns a PDF (Content-Type: application/pdf)
• Same endpoint with a mismatched X-Sender-Address returns 403
• Same endpoint with no auth headers returns 401
• GET /api/accounts/G.../stellar-fees returns base_fee_stroops, xlm_balance, estimated_fee_xlm
• Response includes low_balance: true and top_up_link when XLM balance < 2
• estimateStellarFee(2) returns 0.00002 (2 × 100 stroops / 10 000 000)
• Sending a notification to a user with preferred_language = 'pt' delivers Portuguese text
• Missing translation falls back to English without throwing
• check-notification-templates.ts exits 0 with all templates present; exits 1 if a key is removed

closes #948
closes #949
closes #950

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