Switch Evotor integration to webhook-based token delivery flow

Replace OAuth 2.0 authorization code flow with Evotor's proprietary
webhook token delivery: POST /evotor/callback receives token server-to-server,
GET /evotor/link links it to the logged-in user's account.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -2,6 +2,9 @@ DATABASE_URL=mysql+pymysql://evosync:evosync@db:3306/evosync
 SECRET_KEY=your-random-secret-key-here
 BASE_URL=http://localhost:8000
 
+EVOTOR_APP_ID=your-evotor-app-id
+EVOTOR_WEBHOOK_SECRET=your-webhook-secret
+
 DB_ROOT_PASSWORD=rootpass
 DB_NAME=evosync
 DB_USER=evosync