fix: recover orphaned DeepSeek key + remove retired settings slots

Migration copies ANTHROPIC_API_KEY value → DEEPSEEK_API_KEY (DeepSeek key
was saved before the rename), then deletes ANTHROPIC_API_KEY and
VOYAGE_API_KEY rows. ALLOWED_KEYS now only lists active provider keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPf5Rc8QPx6V8KLEEgfKEQ

app/api/settings/route.ts

@@ -4,7 +4,6 @@ import { NextRequest, NextResponse } from 'next/server'
 import { createClient } from '@/lib/supabase/server'
 
 const ALLOWED_KEYS = [
-  'ANTHROPIC_API_KEY',
   'DEEPSEEK_API_KEY',
   'GOOGLE_AI_API_KEY',
   'META_WHATSAPP_PHONE_NUMBER_ID',