refactor: improve session management and concurrency in Google Search fetcher

- Introduced a semaphore to limit concurrent page renders, enhancing resource management.
- Updated session handling to defer browser closure until all in-flight renders are complete.
- Improved comments for clarity on the behavior of concurrent fetches and session lifecycle.
- Cleaned up imports in alembic environment and migration flow scripts for consistency.

surfsense_backend/scripts/check_migration_flow.py

@@ -94,7 +94,6 @@ async def set_version(version: str | None) -> None:
 
 async def assert_at_head() -> None:
     import asyncpg
-
     from alembic.script import ScriptDirectory
 
     head = ScriptDirectory(str(BACKEND_DIR / "alembic")).get_current_head()