feat(api): item dependency extraction, indexing, and resolve endpoints

- Add Dependency type to internal/kc and extract silo/dependencies.json
  from .kc files on commit
- Create ItemDependencyRepository with ReplaceForRevision, ListByItem,
  and Resolve (LEFT JOIN against items table)
- Add GET /{partNumber}/dependencies and
  GET /{partNumber}/dependencies/resolve endpoints
- Index dependencies in extractKCMetadata with SSE broadcast
- Pack real dependency data into .kc files on checkout
- Update PackInput.Dependencies from []any to []Dependency

Closes #143

internal/api/pack_handlers.go

@@ -52,11 +52,33 @@ func (s *Server) packKCFile(ctx context.Context, data []byte, item *db.Item, rev
 		}
 	}
 
+	// Build dependencies from item_dependencies table.
+	var deps []kc.Dependency
+	dbDeps, err := s.deps.ListByItem(ctx, item.ID)
+	if err != nil {
+		s.logger.Warn().Err(err).Str("part_number", item.PartNumber).Msg("kc: failed to query dependencies for packing")
+	} else {
+		deps = make([]kc.Dependency, len(dbDeps))
+		for i, d := range dbDeps {
+			deps[i] = kc.Dependency{
+				UUID:         d.ChildUUID,
+				PartNumber:   derefStr(d.ChildPartNumber, ""),
+				Revision:     derefInt(d.ChildRevision, 0),
+				Quantity:     derefFloat(d.Quantity, 0),
+				Label:        derefStr(d.Label, ""),
+				Relationship: d.Relationship,
+			}
+		}
+	}
+	if deps == nil {
+		deps = []kc.Dependency{}
+	}
+
 	input := &kc.PackInput{
 		Manifest:     manifest,
 		Metadata:     metadata,
 		History:      history,
-		Dependencies: []any{}, // empty for Phase 2
+		Dependencies: deps,
 	}
 
 	return kc.Pack(data, input)
@@ -95,3 +117,19 @@ func derefStr(p *string, fallback string) string {
 	}
 	return fallback
 }
+
+// derefInt returns the value of a *int pointer, or fallback if nil.
+func derefInt(p *int, fallback int) int {
+	if p != nil {
+		return *p
+	}
+	return fallback
+}
+
+// derefFloat returns the value of a *float64 pointer, or fallback if nil.
+func derefFloat(p *float64, fallback float64) float64 {
+	if p != nil {
+		return *p
+	}
+	return fallback
+}