feat: add weekly smart financial digest

[vantaq-labs]

Summary

• Adds a JWT-protected GET /digest/weekly endpoint for smart weekly financial summaries.
• Computes Monday-Sunday totals, net cash flow, previous-week comparison, category trends, top expenses, upcoming bills, and deterministic highlights.
• Registers the digest route, documents it in OpenAPI/README, and adds focused backend tests.

Bounty claim

/claim #121

Demo

• The endpoint is covered by focused backend tests that build sample users, transactions, recurring bills, and assert the weekly digest response.
• Short demo video: preparing/uploading if the maintainer requires a visual walkthrough; the test command below currently verifies the behavior end-to-end at service/API level.

Test Plan

• cd packages/backend && python3 -m pytest -q tests/test_digest.py → 3 passed

Closes #121

[vantaq-labs]

Local validation update for reviewers:

• python3 -m py_compile packages/backend/app/routes/digest.py passes.
• Implementation commit in this PR: 667e7be.

Happy to adjust quickly if you want a different digest shape or endpoint name.