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

src/notifications/templates/admin_update.txt.j2

@@ -0,0 +1,33 @@
+===============================================
+ANMELDUNGS-AKTUALISIERUNG
+===============================================
+
+Aktualisiert:    {{ updated_date }} um {{ updated_time }} Uhr (UTC)
+Anmelde-ID:      {{ registration_id }}  (Version {{ version }})
+Kind:            {{ child_name }}
+Eltern-E-Mail:   {{ parent_email }}
+
+-----------------------------------------------
+WAS HAT SICH GEÄNDERT
+-----------------------------------------------
+{% for change in changes %}
+  {{ change.field }}:
+    Alt: {{ change.old }}
+    Neu: {{ change.new }}
+{% else %}
+  (keine Änderungen erkannt)
+{% endfor %}
+-----------------------------------------------
+AKTUELLE ANMELDUNG (nach Aktualisierung)
+-----------------------------------------------
+Spielgruppe:     {{ playgroup_types }}
+Tage:            {{ days }}
+Monatl. Beitrag: {{ monthly_fee }}
+
+Elternteil:      {{ parent_name }}
+Adresse:         {{ parent_street }}, {{ parent_postal_code }} {{ parent_city }}
+Telefon:         {{ parent_phone }}
+
+===============================================
+
+Diese Aktualisierung wurde über den automatischen Anmeldeassistenten eingereicht.