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/ruff.toml

@@ -0,0 +1,10 @@
+target-version = "py312"
+line-length = 100
+extend-exclude = ["migrations/versions"]
+
+[lint]
+select = ["E", "F", "W", "I", "B", "UP"]
+ignore = ["E501", "E741", "E702", "B904", "B905", "B007", "F402"]
+
+[lint.per-file-ignores]
+"tests/*" = ["F811"]