feat: add resilient background job retry & monitoring (#130)

[DrGalio]

Summary

Closes #130 — Resilient background job retry & monitoring

Problem

The current run_due endpoint had zero retry logic:

• If send_reminder() returned False or threw an exception, the reminder was still marked sent=True
• No visibility into job health, failure rates, or exhausted jobs
• No way to retry failed reminders

Solution

Retry with Exponential Backoff

Attempt	Wait Before Retry
1st failure	1 minute
2nd failure	5 minutes
3rd failure	25 minutes

After 3 failures, the reminder is marked exhausted (sent=False, retry_count >= 3).

Key Changes

• Model: Added retry_count (INT, default 0) and last_error (VARCHAR 500) columns
• Schema: New columns + partial index idx_reminders_retry for efficient retry queries
• POST /reminders/run: Fixed — only marks sent=True on actual successful delivery
• GET /reminders/stats: New monitoring endpoint with per-channel breakdown
• POST /reminders/:id/retry: Manual reset for exhausted reminders
• Migration: Auto-applied via _ensure_schema_compatibility — zero-downtime deploy
• Tests: 6 new tests (all passing ✅)
• Docs: README updated with retry/monitoring documentation

Monitoring Response

{
  "total": 10, "sent": 7, "pending": 2, "exhausted": 1,
  "retrying": 2, "max_retries": 3,
  "channels": {
    "email": {"sent": 5, "failed_or_pending": 2},
    "whatsapp": {"sent": 2, "failed_or_pending": 1}
  },
  "next_due_at": "2026-03-22T10:00:00"
}

Test Results

tests/test_reminders.py::test_run_due_marks_sent_on_success PASSED
tests/test_reminders.py::test_run_due_retries_on_failure_with_backoff PASSED
tests/test_reminders.py::test_run_due_exhausts_after_max_retries PASSED
tests/test_reminders.py::test_run_due_handles_exception_in_send PASSED
tests/test_reminders.py::test_reminder_stats_endpoint PASSED
tests/test_reminders.py::test_manual_retry_resets_exhausted_reminder PASSED

All existing tests continue to pass (27/28 — 1 pre-existing failure unrelated to this change).

[DrGalio]

Closing duplicate — superseded by #641 which has the complete implementation with tests and OpenAPI docs.