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

workflows/quick-review.yaml

@@ -0,0 +1,19 @@
+workflow:
+  name: quick-review
+  version: 1
+  description: "Single reviewer approval for minor changes"
+
+  states:
+    - draft
+    - pending
+    - approved
+    - rejected
+
+  gates:
+    - role: reviewer
+      label: "Review"
+      required: true
+
+  rules:
+    any_reject: rejected
+    all_required_approve: approved