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/tests/test_mod_api.py

@@ -1,12 +1,11 @@
 """Integration against PostgreSQL in a temporary schema, never touches library rows."""
 import asyncio
-import copy
 import os
 from uuid import uuid4
 
 import httpx
 import pytest
-from sqlalchemy import select, text
+from sqlalchemy import text
 from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 
 from app import db as database
@@ -128,6 +127,7 @@ def test_complete_mod_workflow(pg_manifest, monkeypatch):
                 assert repeated_lines.count("Upstream addition") == 1
                 # A real replacement is converted, and a stale upload is refused.
                 import io
+
                 from PIL import Image
                 image = io.BytesIO()
                 Image.new("RGB", (32, 32), "red").save(image, "PNG")