[Phase 2] Implement transaction endpoints (8 missing)

[creatiVision]

All 8 transaction endpoints are missing:

• transactions/get — List transactions
• transactions/get/id_by_customer — Get single transaction
• transactions/add — Add transaction
• transactions/addBatch — Batch add
• transactions/assign/receipt — Assign receipt to transaction
• transactions/unassign/receipt — Unassign receipt
• transactions/assigned-receipts/get — Get receipts for transaction
• transactions/assign-batch/receipt — Batch assign