test: add regression tests for scheduler service (#1557)

[Srv99x]

Description

Adds robust regression testing for the weekly digest scheduler service to ensure background tasks trigger correctly and handle early-exit conditions gracefully.

Related Issue

Fixes #1557

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Demo Video

Windows.PowerShell.2026-07-27.23-51-42.mp4

Test evidence

$ pytest tests/test_scheduler.py -v
============================= test session starts ==============================
collected 6 items

backend/tests/test_scheduler.py ......                                   [100%]

============================== 6 passed in 2.19s ===============================

[imDarshanGK]

@Srv99x Add demo video showing the working features

[Srv99x]

@imDarshanGK Since this PR specifically adds backend regression tests
for the scheduler service, there is no frontend UI to demonstrate.

Here is the terminal screen recording showing all 6 tests in the
isolated pytest suite executing and passing successfully on Python 3.12:

Windows.PowerShell.2026-07-27.23-51-42.mp4

Test summary:
✅ test_send_weekly_digests_disabled
✅ test_send_weekly_digests_no_subscribers
✅ test_send_weekly_digests_success
✅ test_send_weekly_digests_failure_does_not_update_last_sent_at
✅ test_start_scheduler_adds_job
✅ test_start_scheduler_twice_prevents_duplicate_jobs

6 passed in 2.02s

[Srv99x]

Also, it looks like the GitHub Actions runner for the Python 3.12 backend tests hung and was cancelled after 15 minutes (the Python 3.11 tests passed perfectly in 1m). Could you please re-run that specific failed job when you have a moment to clear the red check? @imDarshanGK

[Srv99x]

@imDarshanGK Just a quick ping on this! I've attached the terminal execution video above as requested. All 11 CI checks are now passing and the branch is up to date with main. Let me know if you need any other changes before merging.