Make email templates runtime-configurable

[ianmuchyri]

Summary

• Verification, password reset, and invitation emails now render from a single .tmpl file each (Subject: + optional Content-Type: + body, minijinja {{ variable }} syntax) instead of hardcoded strings duplicated across identity/service.rs and tenants/email.rs.
• Every template ships a built-in default baked into the image at /app/email-templates; setting ATOM_EMAIL_TEMPLATES_DIR (Compose wires this to a bind mount, defaulting to ./email-templates) lets an operator override any subset at runtime, without rebuilding — useful since Atom is embedded by multiple platforms/projects that each need their own branding.
• Content-Type: text/html is optional per template (defaults to text/plain), so overrides can be full branded HTML.
• Deduplicated the SMTP transport-building code that was previously copy-pasted across all three send functions into src/mail.rs.

Test plan

• cargo test --lib (126 passed)
• cargo fmt --check / cargo clippy --lib -- -D warnings clean
• docker build --target release succeeds and /app/email-templates contains the three defaults
• Verified end-to-end in the portal repo: built atom locally via docker-compose.local-build.yml, mounted branded HTML overrides via ATOM_EMAIL_TEMPLATES_HOST_DIR, confirmed the templates land readable inside the running container