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/tests/unit/platforms/google_search/test_fetch_concurrency.py

@@ -0,0 +1,63 @@
+"""The production regression this guards: several scrape runs render SERPs
+concurrently on the shared browser. When one render failed, `_drop_session`
+closed the browser immediately — under the sibling renders — so every
+in-flight fetch died with TargetClosedError and the runs cascaded into
+"exhausted 24 IPs". The drop must defer the actual close until the last
+in-flight render on that session finishes.
+"""
+
+import asyncio
+
+from app.proprietary.platforms.google_search import fetch
+
+
+class _FakeSession:
+    def __init__(self):
+        self.release = asyncio.Event()
+        self.closed = 0
+
+    async def fetch(self, url, proxy=None):
+        await self.release.wait()
+        return "page"
+
+    async def close(self):
+        self.closed += 1
+
+
+def test_drop_defers_close_until_inflight_renders_finish(monkeypatch):
+    session = _FakeSession()
+
+    async def fake_get_session(mobile):
+        return session
+
+    monkeypatch.setattr(fetch, "_get_session", fake_get_session)
+    monkeypatch.setitem(fetch._sessions, False, session)
+
+    async def main():
+        render = asyncio.create_task(fetch._render_on_loop("u", None, False))
+        await asyncio.sleep(0)  # let the render register as in-flight
+        assert fetch._inflight[session] == 1
+
+        await fetch._drop_session_on_loop(False)
+        assert session.closed == 0, "must not close under an in-flight render"
+        assert session in fetch._doomed
+        assert False not in fetch._sessions  # next fetch relaunches
+
+        session.release.set()
+        assert await render == "page"
+        assert session.closed == 1, "last render out closes the doomed browser"
+        assert session not in fetch._inflight
+        assert session not in fetch._doomed
+
+    asyncio.run(main())
+
+
+def test_drop_closes_immediately_when_idle():
+    session = _FakeSession()
+
+    async def main():
+        fetch._sessions[False] = session
+        await fetch._drop_session_on_loop(False)
+        assert session.closed == 1
+
+    asyncio.run(main())