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>

app/api/schemas/scholars.py

@@ -136,7 +136,7 @@ class DataImportRequest(BaseModel):
 
     @model_validator(mode="before")
     @classmethod
-    def unwrap_export_envelope(cls, values: dict) -> dict:  # type: ignore[override]
+    def unwrap_export_envelope(cls, values: dict) -> dict:
         """Accept the full export envelope format (with data/meta wrapper)."""
         if isinstance(values, dict) and "data" in values and isinstance(values["data"], dict):
             return values["data"]