feat(api): macro indexing from .kc files and read-only API

- Add MacroFile type to internal/kc and extract silo/macros/* files
  from .kc ZIP archives on commit
- Create ItemMacroRepository with ReplaceForItem, ListByItem, and
  GetByFilename methods
- Add GET /{partNumber}/macros (list) and
  GET /{partNumber}/macros/{filename} (source content) endpoints
- Index macros in extractKCMetadata with SSE broadcast
- List endpoint omits content for lightweight responses

Closes #144

internal/api/handlers.go

@@ -57,6 +57,7 @@ type Server struct {
 	settings      *db.SettingsRepository
 	metadata      *db.ItemMetadataRepository
 	deps          *db.ItemDependencyRepository
+	macros        *db.ItemMacroRepository
 }
 
 // NewServer creates a new API server.
@@ -87,6 +88,7 @@ func NewServer(
 	locations := db.NewLocationRepository(database)
 	metadata := db.NewItemMetadataRepository(database)
 	itemDeps := db.NewItemDependencyRepository(database)
+	itemMacros := db.NewItemMacroRepository(database)
 	seqStore := &dbSequenceStore{db: database, schemas: schemas}
 	partgen := partnum.NewGenerator(schemas, seqStore)
 
@@ -117,6 +119,7 @@ func NewServer(
 		settings:      settings,
 		metadata:      metadata,
 		deps:          itemDeps,
+		macros:        itemMacros,
 	}
 }