[codex] Isolate daily summary Redis import

[tianmind-studio]

Summary

• Stub redis.Redis inside test_daily_summary_regenerate.py so the daily summary regenerate tests collect in lightweight Windows backend environments without the Redis SDK installed.
• Restore the real backend/database package path if another stub-heavy test has already left a lightweight database package in sys.modules.
• Populate database._client.db only when a previous test already installed a _client stub, keeping normal imports on the real module path.

Why

test_daily_summary_regenerate.py verifies that update_daily_summary preserves the existing document ID during regeneration. It does not exercise Redis, but importing database.daily_summaries also imports database.redis_db, which imports the optional Redis SDK at module import time. On this Windows lightweight backend venv, collection failed before the regression assertions could run:

ModuleNotFoundError: No module named 'redis'

Running after another lightweight test can also leave a stubbed database package and _client module in sys.modules; the test now restores the package path and fills the existing client stub with the minimal db attribute needed by database.daily_summaries.

Validation

• python -m pytest backend\tests\unit\test_daily_summary_regenerate.py --collect-only -q --tb=short
• python -m pytest backend\tests\unit\test_daily_summary_regenerate.py -q --tb=short
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_daily_summary_regenerate.py -q --tb=short
• python -m pytest backend\tests\unit\test_daily_summary_regenerate.py backend\tests\unit\test_action_item_date_validation.py -q --tb=short
• python -m black --line-length 120 --skip-string-normalization backend\tests\unit\test_daily_summary_regenerate.py
• python -m py_compile backend\tests\unit\test_daily_summary_regenerate.py
• git diff --check
• scripts\pre-commit