Add import system: CSV mapping, progress, rollback, Stripe/Chargebee helpers

[dee-john]

This PR adds a merchant import system to migrate subscriptions from external platforms.

Changes:

• Frontend: src/screens/ImportScreen.tsx — CSV column mapping wizard, progress UI, rollback on failure.
• Backend: backend/services/importService.ts — helpers to fetch/normalize Stripe and Chargebee subscriptions.
• Tests: src/utils/__tests__/importExport.test.ts — unit tests for import utilities.

Acceptance coverage:

• CSV import with column mapping (basic UI)
• API-based import helpers for Stripe and Chargebee
• Import validation using existing validators
• Duplicate detection + upsert/replace logic preserved
• Progress tracking and rollback on failure

Notes:

• Backend helpers are basic and need pagination/error handling for production.
• CI should run tests and catch any environment-specific issues.

Closes: import-system

[Johnjnr1]

closes #473

[Johnjnr1]

This PR adds a merchant import system to migrate subscriptions from external platforms.

Changes:

• Frontend: src/screens/ImportScreen.tsx — CSV column mapping wizard, progress UI, rollback on failure.
• Backend: backend/services/importService.ts — helpers to fetch/normalize Stripe and Chargebee subscriptions.
• Tests: src/utils/__tests__/importExport.test.ts — unit tests for import utilities.

Acceptance coverage:

• CSV import with column mapping (basic UI)
• API-based import helpers for Stripe and Chargebee
• Import validation using existing validators
• Duplicate detection + upsert/replace logic preserved
• Progress tracking and rollback on failure

Notes:

• Backend helpers are basic and need pagination/error handling for production.
• CI should run tests and catch any environment-specific issues.

Closes: import-system

closes #391

[Johnjnr1]

closes #391