Document custom recurrence RRULE patterns and fix stale field names

API.md now lists the supported custom RRULE patterns (every Wednesday,
every other Wednesday, every second Tuesday, every 15th, etc.) matching
the newly implemented dateutil.rrule evaluation. CLAUDE.md's Task model
reference also gets corrected: it listed recurrence_type/recurrence_interval
fields that never existed on the model.

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

CLAUDE.md

@@ -97,7 +97,7 @@ celery -A config beat -l info
 **tasks/** - Core task management
 - Hierarchical tasks with parent/subtask relationships
 - Task status (pending, in_progress, completed, cancelled) and priority (low, medium, high, urgent)
-- Recurrence patterns (daily, weekly, monthly, yearly, custom RRULE)
+- Recurrence patterns (daily, weekly, biweekly, monthly, yearly, or custom RRULE for day-of-week/nth-weekday/day-of-month control)
 - Time tracking with start/stop timer functionality
 - Tag system with colors and icons for organization
 - Task sharing with permission levels (viewer/editor)
@@ -234,7 +234,7 @@ Configured in `config/celery.py`:
 - Hierarchy: `parent` (ForeignKey to self)
 - Status: pending, in_progress, completed, cancelled
 - Priority: low, medium, high, urgent
-- Recurrence: `recurrence_type`, `recurrence_interval`, `recurrence_rule` (RRULE)
+- Recurrence: `recurrence` (none/daily/weekly/biweekly/monthly/yearly/custom), `recurrence_rule` (RRULE string, used when `recurrence='custom'`, parsed via `dateutil.rrule`), `recurrence_end_date`
 - Timing: `due_date`, `due_time`, `completed_at`, `created_at`, `updated_at`
 - Soft delete: `is_deleted` (filters via `SoftDeleteManager`)
 - Sync: `sync_id` (UUID for mobile sync)
@@ -362,7 +362,7 @@ make test                            # Run tests in container
 
 ### Common Development Patterns
 
-**Creating Recurring Tasks:** When a recurring task is marked complete, the next recurrence is automatically created in `tasks/models.py` `create_next_recurrence()` method. Celery task `process_recurring_tasks()` runs as safety net.
+**Creating Recurring Tasks:** When a recurring task is marked complete, the next recurrence is automatically created in `tasks/models.py` `create_next_recurrence()` method, which calls `calculate_next_due_date()`. For `recurrence='custom'`, the next date is computed by evaluating `recurrence_rule` (an RRULE string) via `dateutil.rrule`; the web UI's Custom recurrence builder (in `templates/tasks/_recurrence_fields.html` and `static/js/app.js`) generates these strings for day-of-week, nth-weekday, and day-of-month patterns. Celery task `process_recurring_tasks()` runs as safety net.
 
 **Time Entry Workflow:**
 1. `POST /api/tasks/<id>/start-timer/` creates TimeEntry with start_time