feat: implement reading pace notifications (US #33)

Backend:
- New hermes app with ReadingGroup, GroupMeeting, GroupMembership models
- NotificationPreference (per-user: enable/disable, frequency daily/weekly)
- PaceNotification model for tracking sent/dismissed notifications
- Pace calculation service comparing current progress vs meeting targets
- API endpoints: pace status, notification preferences CRUD, reminders, dismiss
- Ahead/behind/on_track/completed status determination

Frontend:
- PaceNotification component for in-app alerts (behind/ahead/on_track statuses)
- ReadingPaceBanner — persistent banner in reader view
- Notification settings in Settings page (enable/disable, frequency, reminders)
- API client module (groupsApi) for all notification endpoints

backend/apps/groups/serializers.py

@@ -0,0 +1,137 @@
+from __future__ import annotations
+
+from rest_framework import serializers
+
+from apps.groups.models import (
+    FrequencyChoice,
+    GroupMeeting,
+    GroupMembership,
+    NotificationPreference,
+    PaceNotification,
+    ReadingGroup,
+)
+from apps.groups.services.pace import PaceStatus
+
+
+class NotificationPreferenceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = NotificationPreference
+        fields = [
+            "pace_notifications_enabled",
+            "reminder_enabled",
+            "frequency",
+        ]
+
+    def validate_frequency(self, value: str) -> str:
+        valid = {choice[0] for choice in FrequencyChoice.choices}
+        if value not in valid:
+            raise serializers.ValidationError(
+                f"Frequency must be one of: {', '.join(valid)}"
+            )
+        return value
+
+
+class GroupMeetingSerializer(serializers.ModelSerializer):
+    group_name = serializers.CharField(source="group.name", read_only=True)
+
+    class Meta:
+        model = GroupMeeting
+        fields = [
+            "id",
+            "group",
+            "group_name",
+            "title",
+            "scheduled_at",
+            "target_section",
+            "target_section_label",
+            "created_at",
+        ]
+        read_only_fields = ["id", "created_at"]
+
+    def validate_target_section(self, value: int) -> int:
+        if value < 0:
+            raise serializers.ValidationError("Target section must be non-negative.")
+        return value
+
+
+class ReadingGroupSerializer(serializers.ModelSerializer):
+    member_count = serializers.SerializerMethodField()
+    meeting_count = serializers.SerializerMethodField()
+
+    class Meta:
+        model = ReadingGroup
+        fields = [
+            "id",
+            "name",
+            "description",
+            "book",
+            "ebook",
+            "created_by",
+            "created_at",
+            "member_count",
+            "meeting_count",
+        ]
+        read_only_fields = ["id", "created_by", "created_at"]
+
+    def get_member_count(self, obj: ReadingGroup) -> int:
+        return obj.memberships.count()
+
+    def get_meeting_count(self, obj: ReadingGroup) -> int:
+        return obj.meetings.count()
+
+
+class GroupMembershipSerializer(serializers.ModelSerializer):
+    user_email = serializers.CharField(source="user.email", read_only=True)
+    group_name = serializers.CharField(source="group.name", read_only=True)
+
+    class Meta:
+        model = GroupMembership
+        fields = ["id", "user", "user_email", "group", "group_name", "role", "joined_at"]
+        read_only_fields = ["id", "joined_at"]
+
+
+class PaceNotificationSerializer(serializers.ModelSerializer):
+    status_display = serializers.CharField(source="get_status_display", read_only=True)
+
+    class Meta:
+        model = PaceNotification
+        fields = [
+            "id",
+            "user",
+            "meeting",
+            "ebook",
+            "status",
+            "status_display",
+            "current_section",
+            "target_section",
+            "ahead_behind_delta",
+            "created_at",
+        ]
+        read_only_fields = ["id", "created_at"]
+
+
+class PaceStatusSerializer(serializers.Serializer):
+    meeting_id = serializers.IntegerField()
+    meeting_title = serializers.CharField()
+    scheduled_at = serializers.CharField()
+    target_section = serializers.IntegerField()
+    target_section_label = serializers.CharField()
+    current_section = serializers.IntegerField()
+    ahead_behind_delta = serializers.IntegerField()
+    status = serializers.CharField()
+    days_until_meeting = serializers.IntegerField()
+    sections_to_catch_up = serializers.IntegerField()
+    ebook_id = serializers.IntegerField()
+    ebook_title = serializers.CharField()
+
+
+class ReminderSerializer(serializers.Serializer):
+    meeting_id = serializers.IntegerField()
+    meeting_title = serializers.CharField()
+    scheduled_at = serializers.CharField()
+    target_section = serializers.IntegerField()
+    target_section_label = serializers.CharField()
+    group_name = serializers.CharField()
+    hours_until = serializers.IntegerField()
+    ebook_id = serializers.IntegerField()
+    ebook_title = serializers.CharField()