fix: address review -- unused import, React Query, redis import, email placeholder

1. Remove unused 'user_limits' import (CI lint failure)
2. Move redis_client import to module top, remove late import in function
3. Replace real email with admin@example.com in .env.example
4. Convert AdminPage from useEffect+fetch to React Query (useQuery)
   per CLAUDE.md: 'Use React Query for all server data fetching'
   - queryKey: ['admin', 'users']
   - changeTier invalidates query instead of manual state update
   - refetch replaces fetchUsers callback

Skipped: sync->async route conversion. Supabase client is sync,
FastAPI correctly runs sync handlers in threadpool. Converting to
async def with sync DB calls would block the event loop.

flake8 passes. Build passes.

Author: DevanshuNEU

.env.example

@@ -70,4 +70,4 @@ VOYAGE_API_KEY=
 
 # Admin access (comma-separated emails)
 # Users with these emails can access /api/v1/admin/* routes
-ADMIN_EMAILS=devanshurajesh@gmail.com
+ADMIN_EMAILS=admin@example.com

backend/routes/admin.py

@@ -7,7 +7,7 @@
 from fastapi import APIRouter, HTTPException, Depends
 from pydantic import BaseModel
 
-from dependencies import user_limits
+from dependencies import redis_client
 from middleware.auth import require_auth, AuthContext
 from services.observability import logger
 from services.supabase_service import get_supabase_service
@@ -137,7 +137,6 @@ def update_user_tier(
         raise HTTPException(status_code=500, detail="Failed to update tier")
 
     # Clear Redis cache so new tier takes effect immediately
-    from dependencies import redis_client
     if redis_client:
         try:
             redis_client.delete(f"user:tier:{user_id}")

frontend/src/pages/AdminPage.tsx

@@ -5,7 +5,8 @@
  * Admins can change any user's tier with one click.
  */
 
-import { useState, useEffect, useCallback } from 'react'
+import { useState } from 'react'
+import { useQuery, useQueryClient } from '@tanstack/react-query'
 import { Shield, Users, ArrowUpCircle, ArrowDownCircle, Loader2, RefreshCw } from 'lucide-react'
 import { toast } from 'sonner'
 import { Button } from '@/components/ui/button'
@@ -33,33 +34,26 @@ const TIER_COLORS: Record<string, string> = {
 
 export function AdminPage() {
   const { session } = useAuth()
-  const [users, setUsers] = useState<AdminUser[]>([])
-  const [loading, setLoading] = useState(true)
+  const queryClient = useQueryClient()
   const [updating, setUpdating] = useState<string | null>(null)
-  const [error, setError] = useState<string | null>(null)
 
   const headers = { Authorization: `Bearer ${session?.access_token}` }
 
-  const fetchUsers = useCallback(async () => {
-    try {
-      setLoading(true)
-      setError(null)
+  const { data, isLoading: loading, error: queryError, refetch } = useQuery<{ users: AdminUser[] }>({
+    queryKey: ['admin', 'users'],
+    queryFn: async () => {
       const resp = await fetch(`${API_URL}/admin/users`, { headers })
       if (resp.status === 403) {
-        setError('Admin access required. Your email is not in the ADMIN_EMAILS list.')
-        return
+        throw new Error('Admin access required. Your email is not in the ADMIN_EMAILS list.')
       }
       if (!resp.ok) throw new Error('Failed to fetch users')
-      const data = await resp.json()
-      setUsers(data.users || [])
-    } catch (e) {
-      setError(e instanceof Error ? e.message : 'Failed to load users')
-    } finally {
-      setLoading(false)
-    }
-  }, [session?.access_token])
+      return resp.json()
+    },
+    enabled: !!session?.access_token,
+  })
 
-  useEffect(() => { fetchUsers() }, [fetchUsers])
+  const users = data?.users ?? []
+  const error = queryError instanceof Error ? queryError.message : queryError ? 'Failed to load users' : null
 
   async function changeTier(userId: string, email: string, newTier: string) {
     setUpdating(userId)
@@ -73,10 +67,8 @@ export function AdminPage() {
         const err = await resp.json().catch(() => ({}))
         throw new Error(err.detail || 'Failed to update tier')
       }
-      const result = await resp.json()
-      setUsers((prev) =>
-        prev.map((u) => (u.id === userId ? { ...u, tier: result.tier } : u)),
-      )
+      await resp.json()
+      queryClient.invalidateQueries({ queryKey: ['admin', 'users'] })
       toast.success(`Updated ${email} to ${newTier}`)
     } catch (e) {
       toast.error(e instanceof Error ? e.message : 'Update failed')
@@ -90,7 +82,7 @@ export function AdminPage() {
       <div className="flex flex-col items-center justify-center min-h-[400px] gap-4">
         <Shield className="w-12 h-12 text-destructive" />
         <p className="text-muted-foreground">{error}</p>
-        <Button variant="outline" onClick={fetchUsers}>Retry</Button>
+        <Button variant="outline" onClick={() => refetch()}>Retry</Button>
       </div>
     )
   }
@@ -107,7 +99,7 @@ export function AdminPage() {
             <p className="text-sm text-muted-foreground">User management and tier control</p>
           </div>
         </div>
-        <Button variant="outline" onClick={fetchUsers} disabled={loading}>
+        <Button variant="outline" onClick={() => refetch()} disabled={loading}>
           <RefreshCw className={cn('w-4 h-4 mr-2', loading && 'animate-spin')} />
           Refresh
         </Button>