feat: add VK OAuth connection with health checks

- Add VkConnection model with is_online/last_checked_at fields
- Add /vk OAuth flow (connect/callback/disconnect/page)
- Add VK entry to connections dashboard
- Extend background health checker to check VK tokens via users.get
- Add Alembic migration for vk_connections table
- Add VK_CLIENT_ID/SECRET/SCOPES/API_VERSION config settings

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

web/config.py

@@ -13,6 +13,11 @@ class Settings(BaseSettings):
 
     HEALTH_CHECK_INTERVAL_SECONDS: int = 600
 
+    VK_CLIENT_ID: str = ""
+    VK_CLIENT_SECRET: str = ""
+    VK_SCOPES: str = "market groups offline"
+    VK_API_VERSION: str = "5.131"
+
     # Docker compose vars (ignored in app, kept for env compatibility)
     DB_ROOT_PASSWORD: str = ""
     DB_NAME: str = ""