feat: add fake sms backend debugging

[thlaure]

Summary

• add a fake SMS gateway for dev and test so backend alert flows can be debugged without Twilio or real message delivery
• expose a GET /debug/fake-sms inbox endpoint and store fake sends in backend/var/share/fake_sms_inbox.jsonl
• document the fake SMS development workflow in the backend README

Verification

• cd backend && vendor/bin/phpunit --testsuite=Unit
• cd backend && vendor/bin/phpstan analyse --no-progress --memory-limit=-1
• cd backend && vendor/bin/php-cs-fixer fix --dry-run --diff --sequential
• verified app_debug_fake_sms route is registered in the running backend container

Remaining limitations

• Dockerized backend integration tests still hit the pre-existing Symfony framework.test configuration issue
• the messenger container health check is still not addressed in this PR

Manual checklist

• open http://localhost:8002/debug/fake-sms and confirm the fake inbox is reachable in dev
• create an emergency contact in the mobile app and confirm sync still succeeds
• trigger an alert timeout and confirm no real SMS client opens
• verify the rendered message appears in /debug/fake-sms
• verify sms_attempts.provider is fake and the alert status is successful