Consolidate single-action controllers into resource controllers
Merge 11 single-action controllers into their parent resource controllers:
- Invoice: send, sendPreview, clone, changeStatus -> InvoicesController
- Estimate: send, sendPreview, clone, convertToInvoice, changeStatus -> EstimatesController
- Payment: send, sendPreview -> PaymentsController
Extract clone and convert business logic from controllers into services:
- InvoiceService: add clone(), changeStatus()
- EstimateService: add clone(), convertToInvoice(), changeStatus()
Previously this logic was inlined in controllers (~80-90 lines each).
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