Core: Implement Batch Payment Processing for Enterprise

[Smartdevs17]

Description\nImplement batch payment processing for enterprise users to send hundreds of payments in a single transaction. Required for payroll, airdrops, and vendor payments.\n\nCurrent Limitation: Single payments only.\nExpected Outcome: CSV upload, validation, preview, and batch execution with reporting.\n## Acceptance Criteria\n- [ ] CSV template generation\n- [ ] CSV parsing with validation\n- [ ] Payment preview\n- [ ] Duplicate detection\n- [ ] Batch transaction execution\n- [ ] Progress tracking\n- [ ] Batch report generation\n- [ ] Partial failure handling\n## Technical Scope\n- Files: backend/batch, frontend batch upload\n- APIs: /api/v1/batch\n- Edge Cases: Large batches, partial failures\n## Complexity Estimate\n200 points - Validation, processing, reporting.

[DevSolex]

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[drips-wave]

This issue has been completed by @DevSolex as part of the Stellar Wave Program's 4th Wave 🥳

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