fix(database): restore cross_crate_search import in database module facade

The cross_crate_search function exists in database/search.py but was missing
from the database/__init__.py re-exports, causing import failures in
crate_service.py. This restores the import by adding cross_crate_search
to both the import statement and __all__ list.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: Peter

src/docsrs_mcp/database/__init__.py

@@ -59,6 +59,7 @@
 # Re-export from search module
 from .search import (
     _apply_mmr_diversification,
+    cross_crate_search,
     get_see_also_suggestions,
     search_embeddings,
     search_example_embeddings,
@@ -95,6 +96,7 @@
     "store_reexports",
     # Search module
     "_apply_mmr_diversification",
+    "cross_crate_search",
     "get_see_also_suggestions",
     "search_embeddings",
     "search_example_embeddings",