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>

tasks/models.py

@@ -213,9 +213,17 @@ class Task(models.Model):
         elif self.recurrence == 'yearly':
             next_date = base_date + relativedelta(years=1)
         elif self.recurrence == 'custom' and self.recurrence_rule:
-            # TODO: Implement RRULE parsing for custom recurrence
-            # For now, default to weekly
-            next_date = base_date + timedelta(weeks=1)
+            from dateutil.rrule import rrulestr
+            from datetime import datetime, time as dt_time
+
+            dtstart = datetime.combine(base_date, dt_time.min)
+            try:
+                next_occurrence = rrulestr(self.recurrence_rule, dtstart=dtstart).after(dtstart, inc=False)
+            except Exception:
+                return None
+            if not next_occurrence:
+                return None
+            next_date = next_occurrence.date()
         else:
             return None
 
@@ -225,6 +233,65 @@ class Task(models.Model):
 
         return next_date
 
+    @property
+    def parsed_custom_recurrence(self):
+        """
+        Decompose recurrence_rule (an RRULE string) into simple fields for
+        prepopulating the custom recurrence builder UI. Never raises; returns
+        safe defaults for a blank or malformed rule.
+        """
+        result = {
+            'freq': None,
+            'interval': 1,
+            'byweekday': [],
+            'monthly_mode': None,
+            'bymonthday': None,
+            'nth_ordinal': None,
+            'nth_weekday': None,
+        }
+        if not self.recurrence_rule:
+            return result
+
+        import re
+
+        params = {}
+        for part in self.recurrence_rule.split(';'):
+            if '=' in part:
+                key, value = part.split('=', 1)
+                params[key.strip().upper()] = value.strip()
+
+        freq = params.get('FREQ', '').upper()
+        if freq not in ('WEEKLY', 'MONTHLY'):
+            return result
+        result['freq'] = freq.lower()
+
+        try:
+            result['interval'] = int(params.get('INTERVAL', '1'))
+        except ValueError:
+            result['interval'] = 1
+
+        byday = params.get('BYDAY', '')
+
+        if freq == 'WEEKLY':
+            if byday:
+                result['byweekday'] = [d.strip() for d in byday.split(',') if d.strip()]
+        elif freq == 'MONTHLY':
+            bymonthday = params.get('BYMONTHDAY')
+            if bymonthday:
+                try:
+                    result['bymonthday'] = int(bymonthday)
+                    result['monthly_mode'] = 'day'
+                except ValueError:
+                    pass
+            elif byday:
+                match = re.match(r'^(-?\d+)([A-Z]{2})$', byday)
+                if match:
+                    result['nth_ordinal'] = int(match.group(1))
+                    result['nth_weekday'] = match.group(2)
+                    result['monthly_mode'] = 'nth'
+
+        return result
+
     def create_next_recurrence(self):
         """
         Create the next instance of this recurring task.