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

cmd/silod/main.go

@@ -23,6 +23,7 @@ import (
 	"github.com/kindredsystems/silo/internal/modules"
 	"github.com/kindredsystems/silo/internal/schema"
 	"github.com/kindredsystems/silo/internal/storage"
+	"github.com/kindredsystems/silo/internal/workflow"
 	"github.com/rs/zerolog"
 )
 
@@ -235,6 +236,19 @@ func main() {
 		}
 	}
 
+	// Load approval workflow definitions (optional — directory may not exist yet)
+	var workflows map[string]*workflow.Workflow
+	if _, err := os.Stat(cfg.Workflows.Directory); err == nil {
+		workflows, err = workflow.LoadAll(cfg.Workflows.Directory)
+		if err != nil {
+			logger.Fatal().Err(err).Str("directory", cfg.Workflows.Directory).Msg("failed to load workflow definitions")
+		}
+		logger.Info().Int("count", len(workflows)).Msg("loaded workflow definitions")
+	} else {
+		workflows = make(map[string]*workflow.Workflow)
+		logger.Info().Str("directory", cfg.Workflows.Directory).Msg("workflows directory not found, skipping")
+	}
+
 	// Initialize module registry
 	registry := modules.NewRegistry()
 	if err := modules.LoadState(registry, cfg, database.Pool()); err != nil {
@@ -258,7 +272,7 @@ func main() {
 	// Create API server
 	server := api.NewServer(logger, database, schemas, cfg.Schemas.Directory, store,
 		authService, sessionManager, oidcBackend, &cfg.Auth, broker, serverState,
-		jobDefs, cfg.Jobs.Directory, registry, cfg)
+		jobDefs, cfg.Jobs.Directory, registry, cfg, workflows)
 	router := api.NewRouter(server, logger)
 
 	// Start background sweepers for job/runner timeouts (only when jobs module enabled)