fix: i18n JSON files not included in wheel package

Root cause: no __init__.py in i18n/*/ subdirs, so setuptools
packages.find() skipped them. pyproject.toml also lacked
[tool.setuptools.package-data] for *.json glob.

Fixes:
- Add __init__.py to i18n/ and each language subdir (zh/en/jp/ko)
- Add [tool.setuptools.package-data] "*" = ["*.json"]
- Add build/ to .gitignore

Verified: wheel now includes all 4 i18n.json files.

.gitignore

@@ -2,3 +2,4 @@ __pycache__/
 *.pyc
 *.egg-info/
 models/mysql.ddl.sql
+build/

pyproject.toml

@@ -10,3 +10,6 @@ requires-python = ">=3.10"
 
 [tool.setuptools.packages.find]
 include = ["sage_datamart*"]
+
+[tool.setuptools.package-data]
+"*" = ["*.json"]