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/scraping.py

@@ -1,7 +1,5 @@
 """Scraping routes: trigger sync, check status (proxy to ReceiptWitness)."""
 
-from uuid import UUID
-
 from fastapi import APIRouter, Depends, HTTPException, status
 from httpx import HTTPStatusError, RequestError
 
@@ -13,7 +11,7 @@ router = APIRouter(prefix="/scraping", tags=["scraping"])
 
 
 @router.post("/{store_slug}/sync", response_model=SyncTriggerResponse)
-async def trigger_sync(store_slug: str, user_id: UUID = Depends(get_current_user)):
+async def trigger_sync(store_slug: str, user_id: str = Depends(get_current_user)):
     client = ReceiptWitnessClient()
     try:
         result = await client.trigger_sync(str(user_id), store_slug)
@@ -31,7 +29,7 @@ async def trigger_sync(store_slug: str, user_id: UUID = Depends(get_current_user
 
 
 @router.get("/status", response_model=list[SyncStatusResponse])
-async def sync_status(user_id: UUID = Depends(get_current_user)):
+async def sync_status(user_id: str = Depends(get_current_user)):
     client = ReceiptWitnessClient()
     try:
         return await client.get_sync_status(str(user_id))