feat(groups): add rate multipliers management modal

Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.

Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
  user_group_rate_multipliers table (same table as user-side rates)

Frontend:
- New GroupRateMultipliersModal component with:
  - User search and add with email autocomplete
  - Editable rate column with local edit mode (cancel/save)
  - Batch adjust: multiply all rates by a factor
  - Clear all (local operation, requires save to persist)
  - Pagination (10/20/50 per page)
  - Platform icon with brand colors in group info bar
  - Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints

frontend/src/api/admin/groups.ts

@@ -153,6 +153,30 @@ export async function getGroupApiKeys(
   return data
 }
 
+/**
+ * Rate multiplier entry for a user in a group
+ */
+export interface GroupRateMultiplierEntry {
+  user_id: number
+  user_name: string
+  user_email: string
+  user_notes: string
+  user_status: string
+  rate_multiplier: number
+}
+
+/**
+ * Get rate multipliers for users in a group
+ * @param id - Group ID
+ * @returns List of user rate multiplier entries
+ */
+export async function getGroupRateMultipliers(id: number): Promise<GroupRateMultiplierEntry[]> {
+  const { data } = await apiClient.get<GroupRateMultiplierEntry[]>(
+    `/admin/groups/${id}/rate-multipliers`
+  )
+  return data
+}
+
 /**
  * Update group sort orders
  * @param updates - Array of { id, sort_order } objects
@@ -167,6 +191,33 @@ export async function updateSortOrder(
   return data
 }
 
+/**
+ * Clear all rate multipliers for a group
+ * @param id - Group ID
+ * @returns Success confirmation
+ */
+export async function clearGroupRateMultipliers(id: number): Promise<{ message: string }> {
+  const { data } = await apiClient.delete<{ message: string }>(`/admin/groups/${id}/rate-multipliers`)
+  return data
+}
+
+/**
+ * Batch set rate multipliers for users in a group
+ * @param id - Group ID
+ * @param entries - Array of { user_id, rate_multiplier }
+ * @returns Success confirmation
+ */
+export async function batchSetGroupRateMultipliers(
+  id: number,
+  entries: Array<{ user_id: number; rate_multiplier: number }>
+): Promise<{ message: string }> {
+  const { data } = await apiClient.put<{ message: string }>(
+    `/admin/groups/${id}/rate-multipliers`,
+    { entries }
+  )
+  return data
+}
+
 export const groupsAPI = {
   list,
   getAll,
@@ -178,6 +229,9 @@ export const groupsAPI = {
   toggleStatus,
   getStats,
   getGroupApiKeys,
+  getGroupRateMultipliers,
+  clearGroupRateMultipliers,
+  batchSetGroupRateMultipliers,
   updateSortOrder
 }