Add per-task reminder notifications: before due, due now, and overdue

Only proactive notification was the once-a-day 6-7AM digest. Wires up
the previously-unused User.default_reminder_minutes profile setting
and ScheduledReminder model (Gitea #8) to send timely per-task alerts:

- Task.reschedule_reminders(), hooked into Task.save() via a dirty-check
  against due_date/due_time/reminder_at/status/is_deleted, (re)creates up
  to 3 ScheduledReminder rows per active due task: a "before due" reminder
  (from reminder_at if set, else default_reminder_minutes before due), a
  "due now" notification, and an "overdue" notification (mirroring
  is_overdue's day-after rule for date-only due tasks). Hooking into
  save() means every call site - web views, the API, and sync - picks
  this up automatically.
- New Celery task send_scheduled_reminders (beat schedule: every 5 min)
  sends due reminders via whichever of email/push the user has enabled,
  reusing the existing per-user channel toggles, and logs them to the
  Notification table.
- 14 new tests covering the scheduling math, due-date-change/completion/
  deletion cleanup, and the sending task's channel and timing behavior.

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

tasks/models.py

@@ -136,9 +136,18 @@ class Task(models.Model):
         db_table = 'tasks'
         ordering = ['sort_order', '-priority', 'due_date', 'created_at']
 
+    # Fields that affect when/whether reminder notifications should fire.
+    REMINDER_TRACKED_FIELDS = ('due_date', 'due_time', 'reminder_at', 'status', 'is_deleted')
+
     def __str__(self):
         return self.title
 
+    @classmethod
+    def from_db(cls, db, field_names, values):
+        instance = super().from_db(db, field_names, values)
+        instance._loaded_values = dict(zip(field_names, values))
+        return instance
+
     def save(self, *args, **kwargs):
         # Auto-set completed_at when status changes to completed
         if self.status == 'completed' and self.completed_at is None:
@@ -146,7 +155,81 @@ class Task(models.Model):
             self.completed_at = timezone.now()
         elif self.status != 'completed':
             self.completed_at = None
+
+        is_new = self._state.adding
+        loaded_values = getattr(self, '_loaded_values', None)
+        reminder_fields_changed = is_new or loaded_values is None or any(
+            loaded_values.get(field) != getattr(self, field) for field in self.REMINDER_TRACKED_FIELDS
+        )
+
         super().save(*args, **kwargs)
+        self._loaded_values = {field: getattr(self, field) for field in self.REMINDER_TRACKED_FIELDS}
+
+        if reminder_fields_changed:
+            self.reschedule_reminders()
+
+    def _due_and_overdue_moments(self):
+        """
+        Returns (due_moment, overdue_moment) as timezone-aware datetimes in
+        the owner's timezone, or (None, None) if there's nothing to schedule
+        against. Mirrors is_overdue's rule that a date-only due task doesn't
+        flip overdue until the day after due_date.
+        """
+        if not self.due_date:
+            return None, None
+
+        from datetime import datetime, time as dt_time, timedelta
+        from zoneinfo import ZoneInfo
+
+        try:
+            user_tz = ZoneInfo(self.user.timezone)
+        except Exception:
+            user_tz = ZoneInfo('UTC')
+
+        if self.due_time:
+            due_moment = datetime.combine(self.due_date, self.due_time, tzinfo=user_tz)
+            return due_moment, due_moment
+
+        overdue_moment = datetime.combine(self.due_date + timedelta(days=1), dt_time.min, tzinfo=user_tz)
+        return None, overdue_moment
+
+    def reschedule_reminders(self):
+        """
+        Recompute this task's ScheduledReminder rows from its current
+        due_date/due_time/reminder_at/status. Safe to call any time due
+        info changes - clears out not-yet-sent reminders and, if the task
+        is still active and due, schedules fresh ones for "before due"
+        (from reminder_at, or user.default_reminder_minutes before due),
+        "due now", and "overdue".
+        """
+        from datetime import timedelta
+        from notifications.models import ScheduledReminder
+
+        ScheduledReminder.objects.filter(task=self, is_sent=False).delete()
+
+        if self.is_deleted or self.status in ('completed', 'cancelled'):
+            return
+
+        due_moment, overdue_moment = self._due_and_overdue_moments()
+        reminders = []
+
+        if due_moment:
+            if self.reminder_at:
+                before_due_moment = self.reminder_at
+            elif self.user.default_reminder_minutes:
+                before_due_moment = due_moment - timedelta(minutes=self.user.default_reminder_minutes)
+            else:
+                before_due_moment = None
+
+            if before_due_moment:
+                reminders.append(ScheduledReminder(task=self, reminder_type='reminder', remind_at=before_due_moment))
+            reminders.append(ScheduledReminder(task=self, reminder_type='due_soon', remind_at=due_moment))
+
+        if overdue_moment:
+            reminders.append(ScheduledReminder(task=self, reminder_type='overdue', remind_at=overdue_moment))
+
+        if reminders:
+            ScheduledReminder.objects.bulk_create(reminders)
 
     @property
     def is_overdue(self):