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>

api/src/cartsnitch_api/routes/stores.py

@@ -1,7 +1,5 @@
 """Store routes: list stores, manage user store connections."""
 
-from uuid import UUID
-
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -21,7 +19,7 @@ async def list_stores(db: AsyncSession = Depends(get_db)):
 
 @router.get("/me/stores", response_model=list[StoreAccountResponse])
 async def list_user_stores(
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = StoreService(db)
@@ -36,7 +34,7 @@ async def list_user_stores(
 async def connect_store(
     store_slug: str,
     body: ConnectStoreRequest,
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = StoreService(db)
@@ -51,7 +49,7 @@ async def connect_store(
 @router.delete("/me/stores/{store_slug}", status_code=status.HTTP_204_NO_CONTENT)
 async def disconnect_store(
     store_slug: str,
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = StoreService(db)