chore: add CI, lint config, login rate limit and split large modules

- Rate-limit failed logins (8 per 15 min per IP or username, 429 + Retry-After)
- Add ruff/mypy config and GitHub Actions workflow (ruff, pytest, tsc, tests, build)
- Apply ruff autofixes (import order, pyupgrade)
- Extract rpy_helpers.py, engine-util.ts, TranslationParts.tsx and
  translation-types.ts from oversized modules
- Add dependency-free React UI tests
- Refresh README and ARCHITECTURE (Naninovel/Tomie, status and plans)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/app/converter/naninovel.py

@@ -1,8 +1,7 @@
 """Naninovel importer: compiled Unity data -> validated PGVN package."""
-from pathlib import Path, PurePosixPath
 import json
-import re
 import tempfile
+from pathlib import Path, PurePosixPath
 
 from app.blobs import put_bytes, put_file
 from app.converter import assets, locales
@@ -158,8 +157,8 @@ def _convert_package(root: Path, work: Path, progress, package):
 
 def _convert_tomie(root, work, progress, package):
     from app.converter.pipeline import Result, anchors
-    from app.converter.tomie_format import RUNTIME, modules, prepare
     from app.converter.tomie_assets import export_media as export_tomie_media
+    from app.converter.tomie_format import RUNTIME, modules, prepare
 
     native = prepare(package)  # all script checks precede media decoding/blob writes
     scripts = modules(native, package.texts)