fix(email): crash reporting, context limit, schema validation, cachin…

[judikay95]

…g bench

Issue 1 - Crash reporting:

• Add worker_crash_total IntCounterVec to Metrics
• Wrap email worker spawn in monitored restart loop (max 5 restarts, exponential backoff 1s/2s/4s/8s/16s), log FATAL on exhaustion
• Add EmailWorkerDown and EmailWorkerCrashLooping Prometheus alerts

Issue 2 - Template caching:

• Templates already compiled once at EmailTemplateEngine::new() startup
• Add benches/email_templates.rs: cached vs per-request at 1000 renders
• Register bench in Cargo.toml; document restart-to-update policy

Issue 3 - Context size limit:

• Reject contexts > 64 KB (serialised JSON) in render() before Handlebars
• Return structured anyhow error with byte count and template name

Issue 4 - Template schema validation:

• Add templates/schema.json documenting required vars per template
• validate_all_templates() uses realistic per-template fixture data
• Add 16 boundary-value tests: empty strings, special chars, long strings, XSS escaping, and startup validation sanity check

Misc: tighten .gitignore (test snapshots, bench output, temp files)

Description

Type of Change

• Bug fix
• New feature
• Refactor / code cleanup
• Documentation update
• CI / tooling change
• Breaking change

Testing Done

Checklist

• Tests pass locally
• Documentation updated (if applicable)
• No breaking changes, or breaking changes are documented above

Related Issues

Closes #
closes #928
closes #929
closes #930
closes #931

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