feat: Background job retry & monitoring system

[inventelpk-cell]

Summary

Implements a complete background job retry and monitoring system for FinMind, closing #130.

• BackgroundJob model with status tracking (pending/running/completed/failed), JSON payload, retry count, max retries, exponential backoff scheduling, and error capture
• JobExecutionLog model for per-attempt audit trail with timestamps and full error tracebacks
• Job executor engine with automatic retry on failure using exponential backoff (5s, 25s, 125s base^attempt), permanent failure after max retries, and a run_due_jobs() scheduler entry point
• Pluggable handler registry — new job types added via @register_handler("type") decorator
• Admin API endpoints (JWT-protected, ADMIN role required):
  • GET /admin/jobs — paginated listing with status and job_type filters
  • GET /admin/jobs/<id> — job detail with full execution history
  • POST /admin/jobs/<id>/retry — manual retry for permanently failed jobs
• Reminder integration — send_reminder job type wraps existing reminder delivery with automatic retry on failure
• PostgreSQL schema migration for background_jobs and job_execution_logs tables with appropriate indexes
• 18 pytest tests covering the execution engine, admin API, and reminder integration

Acceptance Criteria

• BackgroundJob model with all required fields (id, job_type, status, payload, retry_count, max_retries, last_error, created_at, updated_at, next_retry_at)
• Exponential backoff retry (5s, 25s, 125s)
• Permanent failure after max retries exceeded
• Per-attempt execution logging with timestamps and errors
• Admin endpoints with JWT auth and ADMIN role check
• Pagination and filtering on job list endpoint
• Manual retry endpoint for failed jobs
• Reminder sends wrapped in job framework
• Database schema migration included
• pytest test coverage (18 tests, all passing)
• README updated with new endpoints and documentation

Test plan

• Run pytest tests/test_jobs.py — all 18 tests pass
• Verify existing test suite still passes with Docker Compose (sh scripts/test-backend.sh)
• Manual smoke test: create a job via the service layer, verify retry behavior
• Verify admin endpoints return 403 for non-admin users