Add custom recurrence patterns (day-of-week, nth-weekday, day-of-month)

Recurring tasks were locked to fixed daily/weekly/biweekly/monthly/yearly
intervals. The `custom` recurrence type and `recurrence_rule` field already
existed in the model and API docs, but RRULE evaluation was a TODO stub
that silently fell back to weekly, and no UI exposed the option.

Implements real RRULE parsing via dateutil.rrule, and adds a builder UI
(day-of-week checkboxes for weekly, day-of-month or Nth-weekday for
monthly) so users can express patterns like "every other Wednesday" or
"every second Tuesday" without hand-writing RRULE strings.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/base.html

@@ -140,7 +140,7 @@
     {% endif %}
     {% endblock %}
 
-    <script src="{% static 'js/app.js' %}?v=3"></script>
+    <script src="{% static 'js/app.js' %}?v=4"></script>
     {% block extra_js %}{% endblock %}
 </body>
 </html>