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

frontend/src/api/groups.ts

@@ -0,0 +1,57 @@
+import api from "./client";
+import type {
+  GroupMeeting,
+  MeetingReminder,
+  NotificationPreferences,
+  PaceStatus,
+  ReadingGroup,
+} from "../types/notifications";
+
+export const groupsApi = {
+  async getPaceStatus(ebookId?: number): Promise<PaceStatus[]> {
+    const params: Record<string, string> = {};
+    if (ebookId) params.ebook_id = String(ebookId);
+    const { data } = await api.get<PaceStatus[]>("/groups/notifications/pace/", { params });
+    return data;
+  },
+
+  async getPreferences(): Promise<NotificationPreferences> {
+    const { data } = await api.get<NotificationPreferences>("/groups/notifications/preferences/");
+    return data;
+  },
+
+  async updatePreferences(
+    prefs: Partial<NotificationPreferences>,
+  ): Promise<NotificationPreferences> {
+    const { data } = await api.patch<NotificationPreferences>(
+      "/groups/notifications/preferences/",
+      prefs,
+    );
+    return data;
+  },
+
+  async dismissPaceNotification(meetingId: number, ebookId: number): Promise<void> {
+    await api.post("/groups/notifications/dismiss/", {
+      meeting_id: meetingId,
+      ebook_id: ebookId,
+    });
+  },
+
+  async getReminders(): Promise<MeetingReminder[]> {
+    const { data } = await api.get<MeetingReminder[]>("/groups/notifications/reminders/");
+    return data;
+  },
+
+  async getGroups(): Promise<ReadingGroup[]> {
+    const { data } = await api.get<{ count: number; results: ReadingGroup[] } | ReadingGroup[]>(
+      "/groups/groups/",
+    );
+    if (Array.isArray(data)) return data;
+    return data.results ?? [];
+  },
+
+  async getUpcomingMeetings(): Promise<GroupMeeting[]> {
+    const { data } = await api.get<GroupMeeting[]>("/groups/meetings/upcoming/");
+    return data;
+  },
+};