Add alert-on-failure wrapper for Render cron jobs

[jonschull]

Summary

• Addresses Audit: identify processes that fail silently without alerting #579
• Render crons silently swallow non-zero exit codes — nobody finds out when a job fails
• Adds cron_alert.py wrapper that catches unhandled exceptions and sends an alert email
• Wired into all 4 Render cron entry points: map-email-cron, bigmap-screenshot-cron, fathom-sync, generate-followups

Test plan

• Wrapper doesn't affect normal execution (pass-through on success)
• Alert email is best-effort — failure to send alert doesn't mask the real error
• Verify alert email arrives when a cron job fails on Render

🤖 Generated with Claude Code

[claude-ERA]

Auto-merged

PR #583 merged and deployed.

Smoke test: WARNING — see output

Verify: https://era-website.onrender.com

Test output

Traceback (most recent call last):
  File "/Users/admin/era2c/era2/website/tests/smoke_test.py", line 12, in <module>
    from playwright.sync_api import sync_playwright
ModuleNotFoundError: No module named 'playwright'