Add VK OAuth implicit flow and fix sync issues

- Replace manual community token entry with OAuth button that redirects
  to VK authorization and auto-saves token via /vk/callback
- Fix groups.get API call (was groups.getById) to correctly retrieve
  admin group id and name from user token response
- Fix price comparison: VK price.amount is in roubles, not kopecks
- Keep manual token input as fallback when VK_CLIENT_ID is not set

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

web/config.py

@@ -18,6 +18,8 @@ class Settings(BaseSettings):
 
     VK_DEFAULT_PHOTO_PATH: str = "/app/default_product.png"
 
+    VK_CLIENT_ID: str = ""
+    VK_CLIENT_SECRET: str = ""
     VK_API_VERSION: str = "5.131"
 
     # Docker compose vars (ignored in app, kept for env compatibility)