Add weekly financial digest endpoint

[darshan-Jahagirdar]

Summary

• Add GET /insights/weekly-digest for authenticated users
• Summarize weekly income, expenses, net flow, prior-week comparison, top categories, trend insights, and recommended actions
• Keep the digest deterministic and usable without paid AI API keys
• Add focused tests and README documentation

Closes #121

Validation

• From packages/backend: .\.venv-backend-test\Scripts\python.exe -m pytest tests\test_insights.py -q -k weekly_digest -> 2 passed, 3 deselected
• From packages/backend: .\.venv-backend-test\Scripts\python.exe -m black --check app\services\digests.py app\routes\insights.py tests\test_insights.py
• From packages/backend: .\.venv-backend-test\Scripts\python.exe -m flake8 app\services\digests.py app\routes\insights.py tests\test_insights.py
• python -m py_compile packages\backend\app\services\digests.py packages\backend\app\routes\insights.py packages\backend\tests\test_insights.py
• git diff --check

Note: the new digest tests mint JWTs directly so they can run locally without Redis; the existing auth fixture in this repo still requires Redis for full-module runs.

/claim #121

[darshan-Jahagirdar]

Validation update for this PR:

python -m compileall -q packages\backend\app
git diff --check

Result: both passed locally.

I also tried the full backend pytest suite in a temporary Python 3.11 virtualenv with packages/backend/requirements.txt installed. That run is blocked in this local environment because auth-dependent tests try to connect to Redis at redis:6379, which is not resolvable here.