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/routes/shopping.py

@@ -1,7 +1,5 @@
 """Shopping routes: optimize list, saved lists."""
 
-from uuid import UUID
-
 from fastapi import APIRouter, Depends, HTTPException, status
 from httpx import HTTPStatusError, RequestError
 
@@ -13,7 +11,7 @@ router = APIRouter(prefix="/shopping", tags=["shopping"])
 
 
 @router.post("/optimize", response_model=OptimizeResponse)
-async def optimize_shopping(body: OptimizeRequest, user_id: UUID = Depends(get_current_user)):
+async def optimize_shopping(body: OptimizeRequest, user_id: str = Depends(get_current_user)):
     client = ClipArtistClient()
     try:
         result = await client.optimize(
@@ -37,7 +35,7 @@ async def optimize_shopping(body: OptimizeRequest, user_id: UUID = Depends(get_c
 
 
 @router.get("/lists", response_model=list[ShoppingListResponse])
-async def list_shopping_lists(user_id: UUID = Depends(get_current_user)):
+async def list_shopping_lists(user_id: str = Depends(get_current_user)):
     client = ClipArtistClient()
     try:
         return await client.get_shopping_lists(str(user_id))