Add retry monitoring for reminder jobs

[darshan-Jahagirdar]

Summary

• Add bounded retry handling for due reminder delivery jobs
• Keep failed reminders unsent, track cumulative delivery attempts, and store last_error
• Add Prometheus background job counters for attempts, retries, successes, and failures
• Return processed, sent, failed, and retries from /reminders/run
• Document retry configuration and monitoring behavior

Closes #130

Validation

• py -3.11 -m venv .venv-backend-test
• .\.venv-backend-test\Scripts\python.exe -m pip install -r packages\backend\requirements.txt
• From packages/backend: ..\..\.venv-backend-test\Scripts\python.exe -m pytest tests\test_reminders.py -q -k run_due -> 2 passed, 2 deselected
• From packages/backend: ..\..\.venv-backend-test\Scripts\python.exe -m black --check app\config.py app\__init__.py app\models.py app\observability.py app\services\jobs.py app\routes\reminders.py tests\test_reminders.py
• From packages/backend: ..\..\.venv-backend-test\Scripts\python.exe -m flake8 app\config.py app\__init__.py app\models.py app\observability.py app\services\jobs.py app\routes\reminders.py tests\test_reminders.py
• python -m py_compile packages\backend\app\config.py packages\backend\app\__init__.py packages\backend\app\models.py packages\backend\app\observability.py packages\backend\app\services\jobs.py packages\backend\app\routes\reminders.py packages\backend\tests\test_reminders.py
• git diff --check

Note: running the entire existing tests/test_reminders.py module locally requires Redis for the pre-existing auth_header fixture; the new retry tests avoid Redis by minting JWTs directly and passed in the local environment.

/claim #130

[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.