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_ratelimit.py

@@ -0,0 +1,25 @@
+from app.ratelimit import FailureLimiter
+
+
+def test_blocks_after_max_failures_and_resets():
+    limiter = FailureLimiter(max_failures=3, window_seconds=60)
+    for _ in range(2):
+        limiter.fail("ip:1", "user:a")
+    assert limiter.retry_after("ip:1", "user:a") == 0
+    limiter.fail("ip:1", "user:a")
+    assert limiter.retry_after("ip:1", "user:a") > 0
+    assert limiter.retry_after("ip:2", "user:b") == 0
+    limiter.reset("ip:1", "user:a")
+    assert limiter.retry_after("ip:1", "user:a") == 0
+
+
+def test_window_expires(monkeypatch):
+    import app.ratelimit as rl
+
+    now = [1000.0]
+    monkeypatch.setattr(rl.time, "monotonic", lambda: now[0])
+    limiter = FailureLimiter(max_failures=1, window_seconds=10)
+    limiter.fail("k")
+    assert limiter.retry_after("k") > 0
+    now[0] += 11
+    assert limiter.retry_after("k") == 0