refactor notifications to MVC: extract templates, context, and renderer

- Move all email layout to Jinja2 templates (4 files: admin_new,
  admin_update, parent_confirmation html+txt)
- Extract pure helper/context-builder functions to context.py
  (format_types, calculate_age, format_dob, calculate_monthly_fee,
  format_days, build_admin_new_context, build_admin_update_context,
  build_parent_context)
- Move i18n label strings out of Python into YAML locale files
  (i18n/de.yaml, i18n/en.yaml) — admin-editable without code changes
- Add renderer.py as thin Jinja2 wrapper (render_template)
- Reduce notifier.py to routing + SMTP dispatch only
- Update tests to call context functions directly and render
  templates via renderer instead of calling private builder methods
- Add jinja2 and pyyaml as explicit dependencies in pyproject.toml

https://claude.ai/code/session_01LjjK7RjKVnC8bETtccfgna

pyproject.toml

@@ -13,6 +13,8 @@ dependencies = [
     "chainlit>=2.9.6",
     "qrbill>=1.2.0",
     "pillow>=12.1.1",
+    "jinja2>=3.0.0",
+    "pyyaml>=6.0.0",
 ]
 
 [project.scripts]