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/prices.py

@@ -20,7 +20,7 @@ router = APIRouter(prefix="/prices", tags=["prices"])
 
 @router.get("/trends", response_model=list[PriceTrendResponse])
 async def price_trends(
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     category: str | None = Query(None),
     db: AsyncSession = Depends(get_db),
 ):
@@ -30,7 +30,7 @@ async def price_trends(
 
 @router.get("/increases", response_model=list[PriceIncreaseResponse])
 async def price_increases(
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = PriceService(db)
@@ -40,7 +40,7 @@ async def price_increases(
 @router.get("/comparison", response_model=list[PriceComparisonResponse])
 async def price_comparison(
     product_ids: Annotated[list[UUID], Query()],
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = PriceService(db)