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/handlers.go

@@ -56,6 +56,7 @@ type Server struct {
 	cfg           *config.Config
 	settings      *db.SettingsRepository
 	metadata      *db.ItemMetadataRepository
+	deps          *db.ItemDependencyRepository
 }
 
 // NewServer creates a new API server.
@@ -85,6 +86,7 @@ func NewServer(
 	settings := db.NewSettingsRepository(database)
 	locations := db.NewLocationRepository(database)
 	metadata := db.NewItemMetadataRepository(database)
+	itemDeps := db.NewItemDependencyRepository(database)
 	seqStore := &dbSequenceStore{db: database, schemas: schemas}
 	partgen := partnum.NewGenerator(schemas, seqStore)
 
@@ -114,6 +116,7 @@ func NewServer(
 		cfg:           cfg,
 		settings:      settings,
 		metadata:      metadata,
+		deps:          itemDeps,
 	}
 }