fix(api): replace UUID type with str for Better-Auth nanoid user IDs (#98)

Better-Auth uses nanoid strings for user IDs, not UUIDs. Changed all
user_id parameter/return types in the API layer from UUID to str,
removed the obsolete UUID import where unused, and updated the
_validate_session_token return type accordingly.

Co-authored-by: CartSnitch Engineer Bot <cartnoreply@cartsnitch.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/services/stores.py

@@ -1,7 +1,6 @@
 """Store service — list stores, manage user store account connections."""
 
 import json
-from uuid import UUID
 
 from cryptography.fernet import Fernet
 from sqlalchemy import select
@@ -35,7 +34,7 @@ class StoreService:
             for s in stores
         ]
 
-    async def list_user_stores(self, user_id: UUID) -> list[dict]:
+    async def list_user_stores(self, user_id: str) -> list[dict]:
         from cartsnitch_api.models import UserStoreAccount
 
         result = await self.db.execute(
@@ -60,7 +59,7 @@ class StoreService:
             for a in accounts
         ]
 
-    async def connect_store(self, user_id: UUID, store_slug: str, credentials: dict | None) -> dict:
+    async def connect_store(self, user_id: str, store_slug: str, credentials: dict | None) -> dict:
         from cartsnitch_api.models import Store, UserStoreAccount
 
         result = await self.db.execute(select(Store).where(Store.slug == store_slug))
@@ -107,7 +106,7 @@ class StoreService:
             "sync_status": "active",
         }
 
-    async def disconnect_store(self, user_id: UUID, store_slug: str) -> None:
+    async def disconnect_store(self, user_id: str, store_slug: str) -> None:
         from cartsnitch_api.models import Store, UserStoreAccount
 
         result = await self.db.execute(select(Store).where(Store.slug == store_slug))