Fix SQLite timestamp, UUID, and User.id binding in test fixtures

Builds on the partial bd6b137 fix (which only stripped server_default
expressions) by also:

- Add _StringUUID TypeDecorator: lets Text/String/UUID columns accept
  uuid.UUID values on bind (SQLite has no native UUID type) and returns
  uuid.UUID on read so existing test assertions like
  isinstance(store.id, uuid.UUID) still pass.

- Replace UUID column types with _StringUUID before create_all so
  CREATE TABLE uses CHAR(36) instead of the native UUID type that
  SQLite can't bind.

- Extend before_insert listener to also set Text PK columns (User.id)
  and func.now()-stripped columns (ingested_at) to Python-side defaults
  so INSERTs without explicit values succeed under SQLite.

- Switch _create_test_user_and_session to use 32-char hex user/session
  ids so they match the format bound by the TypeDecorator on FK reads.

- Simplify test_encrypted_json.py to use the shared engine/session
  fixtures from conftest instead of duplicating its own broken engine.

Tests passing: tests/test_models.py (14), tests/test_encrypted_json.py (6).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_encrypted_json.py

@@ -5,42 +5,13 @@ import json
 import pytest
 from cryptography.fernet import Fernet
 from pydantic import ValidationError
-from sqlalchemy import column, create_engine, table, text
-from sqlalchemy.orm import sessionmaker
+from sqlalchemy import column, table, text
 
 from cartsnitch_api.config import settings
-from cartsnitch_api.models import Base
 from cartsnitch_api.models.store import Store
 from cartsnitch_api.models.user import User, UserStoreAccount
 
 
-@pytest.fixture
-def engine():
-    eng = create_engine("sqlite:///:memory:")
-
-    for tbl in Base.metadata.tables.values():
-        for col in tbl.columns.values():
-            sd = col.server_default
-            if sd is not None:
-                if not hasattr(sd, "expression"):
-                    col.server_default = None
-                    continue
-                expr_str = str(sd.expression).lower()
-                if "gen_random_uuid" in expr_str or "gen_random_bytes" in expr_str:
-                    col.server_default = None
-
-    Base.metadata.create_all(eng)
-    yield eng
-    eng.dispose()
-
-
-@pytest.fixture
-def session(engine):
-    factory = sessionmaker(bind=engine)
-    with factory() as sess:
-        yield sess
-
-
 @pytest.fixture
 def store(session):
     s = Store(name="Test Store", slug="test-store")