Document reminders, offline PWA, and fix missing Celery Beat in deploy guide

API.md: document the reminder/due_soon/overdue notification schedule
(including the due_time-aware is_overdue fix and the 1-hour overdue
delay) and how default_reminder_minutes/reminder_at interact.

README.md: add Offline Support (PWA) and Notifications & Reminders to
the feature list - both were fully functional but never listed. Also
fixes a real gap in the manual systemd deployment guide: it only set
up the Celery worker, never Celery Beat, so the daily digest, recurring
task safety net, and now reminders would never fire for anyone
following that guide.

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

API.md

@@ -433,7 +433,7 @@ Creates a new task.
 | priority | enum | No | `low`, `medium`, `high`, `urgent` |
 | due_date | date | No | YYYY-MM-DD |
 | due_time | time | No | HH:MM:SS |
-| reminder_at | datetime | No | ISO 8601 format |
+| reminder_at | datetime | No | ISO 8601 format. Overrides the user's `default_reminder_minutes` for this task's "before due" reminder - see Notifications |
 | recurrence | enum | No | `none`, `daily`, `weekly`, `biweekly`, `monthly`, `yearly`, `custom` |
 | recurrence_rule | string | No | RRULE string for custom recurrence |
 | recurrence_end_date | date | No | End date for recurring tasks |
@@ -469,7 +469,7 @@ Creates a new task.
 **Notes:**
 - When a recurring task is completed, the next instance is automatically created
 - `completed_at` is automatically set when `status` changes to `completed`
-- `is_overdue` is calculated based on user's timezone
+- `is_overdue` is calculated in the user's timezone; if `due_time` is set, the task becomes overdue once that time passes on `due_date`, otherwise it becomes overdue starting the day after `due_date`
 
 ### Get Task Details
 
@@ -949,6 +949,13 @@ Returns notifications for current user.
 - `shared` - Task/tag shared with user
 - `daily_email` - Daily email digest sent
 
+**Reminder scheduling:** For any task with `due_date` and `due_time` set, up to three notifications are sent automatically (delivered by email and/or push per the user's `email_notifications`/`push_notifications` settings):
+- `reminder` - before due, at `reminder_at` if set on the task, otherwise `default_reminder_minutes` before the due moment
+- `due_soon` - exactly at the due moment
+- `overdue` - one hour after the due moment (kept separate from `due_soon` so they don't arrive together)
+
+A task with `due_date` but no `due_time` only gets the `overdue` notification, fired at the start of the day after `due_date` (matching `is_overdue`'s day-after rule). Changing `default_reminder_minutes` retroactively reschedules the `reminder` notification on existing active tasks that don't have their own `reminder_at` override; it does not affect tasks whose `reminder_at` was explicitly set.
+
 ### Mark Notification as Read
 
 **POST** `/api/notifications/{notification_id}/read/` (Authenticated)