feat: add tenant_context module — ContextVar, transaction listener, tenant_filter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_tenant_context.py

@@ -0,0 +1,43 @@
+import asyncio
+from uuid import UUID
+import pytest
+from unittest.mock import MagicMock
+
+from app.core.tenant_context import (
+    set_current_account_id,
+    clear_current_account_id,
+    get_current_account_id,
+)
+
+
+def test_contextvar_is_none_by_default():
+    assert get_current_account_id() is None
+
+
+def test_set_and_clear():
+    account_id = UUID("aaaaaaaa-0000-0000-0000-000000000001")
+    token = set_current_account_id(account_id)
+    assert get_current_account_id() == str(account_id)
+    clear_current_account_id(token)
+    assert get_current_account_id() is None
+
+
+def test_tasks_are_isolated():
+    """Each asyncio task has its own ContextVar value."""
+    results = {}
+
+    async def set_in_task(name: str, value: str):
+        token = set_current_account_id(UUID(value))
+        await asyncio.sleep(0)
+        results[name] = get_current_account_id()
+        clear_current_account_id(token)
+
+    async def run():
+        await asyncio.gather(
+            set_in_task("a", "aaaaaaaa-0000-0000-0000-000000000001"),
+            set_in_task("b", "bbbbbbbb-0000-0000-0000-000000000002"),
+        )
+
+    asyncio.run(run())
+    assert results["a"] == "aaaaaaaa-0000-0000-0000-000000000001"
+    assert results["b"] == "bbbbbbbb-0000-0000-0000-000000000002"