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

internal/config/config.go

@@ -10,15 +10,16 @@ import (
 
 // Config holds all application configuration.
 type Config struct {
-	Server   ServerConfig   `yaml:"server"`
-	Database DatabaseConfig `yaml:"database"`
-	Storage  StorageConfig  `yaml:"storage"`
-	Schemas  SchemasConfig  `yaml:"schemas"`
-	FreeCAD  FreeCADConfig  `yaml:"freecad"`
-	Odoo     OdooConfig     `yaml:"odoo"`
-	Auth     AuthConfig     `yaml:"auth"`
-	Jobs     JobsConfig     `yaml:"jobs"`
-	Modules  ModulesConfig  `yaml:"modules"`
+	Server    ServerConfig    `yaml:"server"`
+	Database  DatabaseConfig  `yaml:"database"`
+	Storage   StorageConfig   `yaml:"storage"`
+	Schemas   SchemasConfig   `yaml:"schemas"`
+	FreeCAD   FreeCADConfig   `yaml:"freecad"`
+	Odoo      OdooConfig      `yaml:"odoo"`
+	Auth      AuthConfig      `yaml:"auth"`
+	Jobs      JobsConfig      `yaml:"jobs"`
+	Workflows WorkflowsConfig `yaml:"workflows"`
+	Modules   ModulesConfig   `yaml:"modules"`
 }
 
 // ModulesConfig holds explicit enable/disable toggles for optional modules.
@@ -146,6 +147,11 @@ type JobsConfig struct {
 	DefaultPriority int    `yaml:"default_priority"`  // default 100
 }
 
+// WorkflowsConfig holds approval workflow definition settings.
+type WorkflowsConfig struct {
+	Directory string `yaml:"directory"` // default /etc/silo/workflows
+}
+
 // OdooConfig holds Odoo ERP integration settings.
 type OdooConfig struct {
 	Enabled  bool   `yaml:"enabled"`
@@ -204,6 +210,9 @@ func Load(path string) (*Config, error) {
 	if cfg.Jobs.DefaultPriority == 0 {
 		cfg.Jobs.DefaultPriority = 100
 	}
+	if cfg.Workflows.Directory == "" {
+		cfg.Workflows.Directory = "/etc/silo/workflows"
+	}
 
 	// Override with environment variables
 	if v := os.Getenv("SILO_DB_HOST"); v != "" {