Replace VK OAuth with manual community token entry

Resolves #4 — VK OAuth flow caused "Security Error" because market sync
requires a community access token, not a personal user token. Replaced
OAuth with manual token input (same pattern as Evotor). Added
step-by-step instructions. Updated health checker to validate community
tokens via groups.getById instead of users.get.

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

web/health_checker.py

@@ -11,7 +11,7 @@ logger = logging.getLogger("uvicorn.error")
 
 EVOTOR_STORES_URL = "https://api.evotor.ru/stores"
 EVOTOR_TOKEN_URL = "https://oauth.evotor.ru/oauth/token"
-VK_USERS_GET_URL = "https://api.vk.com/method/users.get"
+VK_GROUPS_GET_URL = "https://api.vk.com/method/groups.getById"
 VK_API_VERSION = "5.131"
 
 # Refresh Evotor token if it expires within this window
@@ -59,7 +59,7 @@ async def check_vk_connection(access_token: str) -> bool:
     try:
         async with httpx.AsyncClient() as client:
             resp = await client.get(
-                VK_USERS_GET_URL,
+                VK_GROUPS_GET_URL,
                 params={"access_token": access_token, "v": VK_API_VERSION},
                 timeout=10,
             )