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/integration/test_fixture_probe_runs.py

@@ -76,6 +76,9 @@ class FixtureScholarSource:
     async def fetch_profile_html(self, scholar_id: str) -> FetchResult:
         return await self.fetch_profile_page_html(scholar_id, cstart=0, pagesize=100)
 
+    async def fetch_author_search_html(self, query: str, *, start: int = 0) -> FetchResult:
+        raise NotImplementedError
+
 
 @pytest.mark.integration
 @pytest.mark.db