feat: VK OAuth flow, catalog sync improvements, and expanded test suite

- Add VK OAuth implicit flow: /vk-auth redirect, /vk-callback JS page,
  /vk-callback/save endpoint with state validation
- Add VK_CLIENT_ID/VK_CLIENT_SECRET to config
- Add refresh_token/token_expires_at columns to vk_connections (migration 0006)
- Fix vk_catalog task: handle price/thumb_photo as string or dict (VK API v5.199)
- Fix connections/vk/test: use groups.getById instead of market.getAlbums
  (works with both user and group tokens)
- Add orphan deletion to mirror_to_vk: VK products not in Evotor are removed
- Handle ungrouped Evotor products: push to "Без категории" VK album
- Respect SyncConfig.is_enabled in mirror_to_vk
- Add product count column to catalog groups page
- Add group name column to catalog products page
- Expand test suite: 73 new tests covering connections routes, catalog routes,
  vk_sync task logic, and catalog task helpers (138 total, all passing)

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

web/tasks/vk_catalog.py

@@ -91,13 +91,20 @@ def _sync_user(db, user_id: int, token: str, group_id: str) -> None:
     for p in all_products:
         pid = str(p["id"])
         album_id = str(p["albums_ids"][0]) if p.get("albums_ids") else None
-        price_raw = p.get("price", {}).get("amount")
-        price = float(price_raw) / 100 if price_raw is not None else None
+        price_field = p.get("price")
+        if isinstance(price_field, dict):
+            price_raw = price_field.get("amount")
+            price = float(price_raw) / 100 if price_raw is not None else None
+        else:
+            price = float(price_field) / 100 if price_field is not None else None
         thumb = None
-        if p.get("thumb_photo"):
-            sizes = p["thumb_photo"].get("sizes", [])
+        thumb_field = p.get("thumb_photo")
+        if isinstance(thumb_field, dict):
+            sizes = thumb_field.get("sizes", [])
             if sizes:
                 thumb = sizes[-1].get("url")
+        elif isinstance(thumb_field, str):
+            thumb = thumb_field
 
         row = db.query(VkCachedProduct).filter_by(
             user_id=user_id, vk_group_id=group_id, vk_product_id=pid,