Fix SQLite CI test failures: UUID binding, func.now() defaults, F402 lint

Three fixes from PR #35 review:

1. Fix F402: rename loop var 'table' → 'metadata_table' in test_encrypted_json.py
2. Strip func.now() server_defaults in conftest.py engine/db_engine fixtures
3. Add aiosqlite UUID adapter for async engine

Model changes to provide Python-side defaults for SQLite compatibility:
- TimestampMixin: add default=_utcnow for created_at/updated_at
- UUIDPrimaryKeyMixin: use GuidType for cross-DB UUID handling
- User.id: use GuidType() instead of Text, Mapped[uuid.UUID]
- User.email_verified: add default=False
- Purchase.ingested_at: add default=_utcnow
- types.py: add GuidType TypeDecorator for UUID→String conversion

Fixes: CAR-1012

tests/test_encrypted_json.py

@@ -18,15 +18,16 @@ from cartsnitch_api.models.user import User, UserStoreAccount
 def engine():
     eng = create_engine("sqlite:///:memory:")
 
-    for table in Base.metadata.tables.values():
-        for col in table.columns.values():
+    for metadata_table in Base.metadata.tables.values():
+        for col in metadata_table.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:
+                _pg_fns = ("gen_random_uuid", "gen_random_bytes", "now()")
+                if any(pg_fn in expr_str for pg_fn in _pg_fns):
                     col.server_default = None
 
     Base.metadata.create_all(eng)