fix: resolve mypy errors, fix arxiv test session isolation, and add premerge script

- Remove unused type:ignore comment in scholars.py
- Fix 51 mypy errors across test files (Any return types, cast(), protocol stubs)
- Fix arxiv unit test failures caused by session isolation (patch_session_factory fixture)
- Fix integration test rate-limit bleed between search and create
- Add scripts/premerge.sh for local CI verification
- Add docs/website/.docusaurus/ to .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/services/domains/arxiv/conftest.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+import pytest
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
+
+
+@pytest.fixture
+def patch_session_factory(db_session: AsyncSession, monkeypatch: pytest.MonkeyPatch):
+    """Patch get_session_factory in arxiv modules to use the test's DB engine.
+
+    Without this, the implementation creates its own engine via get_session_factory()
+    which may not share transaction visibility with the test's db_session fixture.
+    """
+    factory = async_sessionmaker(db_session.bind, expire_on_commit=False)
+    monkeypatch.setattr("app.services.arxiv.rate_limit.get_session_factory", lambda: factory)
+    monkeypatch.setattr("app.services.arxiv.cache.get_session_factory", lambda: factory)
+    return factory