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

backend/internal/server/routes/admin.go

@@ -228,6 +228,9 @@ func registerGroupRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		groups.PUT("/:id", h.Admin.Group.Update)
 		groups.DELETE("/:id", h.Admin.Group.Delete)
 		groups.GET("/:id/stats", h.Admin.Group.GetStats)
+		groups.GET("/:id/rate-multipliers", h.Admin.Group.GetGroupRateMultipliers)
+		groups.PUT("/:id/rate-multipliers", h.Admin.Group.BatchSetGroupRateMultipliers)
+		groups.DELETE("/:id/rate-multipliers", h.Admin.Group.ClearGroupRateMultipliers)
 		groups.GET("/:id/api-keys", h.Admin.Group.GetGroupAPIKeys)
 	}
 }