feat(api): approvals + ECO workflow API with YAML-configurable workflows
- Add internal/workflow/ package for YAML workflow definitions (Load, LoadAll, Validate)
- Add internal/db/item_approvals.go repository (Create, AddSignature, GetWithSignatures, ListByItemWithSignatures, UpdateState, UpdateSignature)
- Add internal/api/approval_handlers.go with 4 endpoints:
- GET /{partNumber}/approvals (list approvals with signatures)
- POST /{partNumber}/approvals (create ECO with workflow + signers)
- POST /{partNumber}/approvals/{id}/sign (approve or reject)
- GET /workflows (list available workflow definitions)
- Rule-driven state transitions: any_reject and all_required_approve
- Pack approvals into silo/approvals.json on .kc checkout
- Add WorkflowsConfig to config, load workflows at startup
- Migration 019: add workflow_name column to item_approvals
- Example workflows: engineering-change.yaml, quick-review.yaml
- 7 workflow tests, all passing
Closes #145
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