fix: resolve CI failures — SQLite incompatibility and ruff lint errors

- Remove PostgreSQL-specific server_default from User.email_inbound_token.
  The column has a Python-side default (secrets.token_urlsafe) that works
  for both SQLite and PostgreSQL. The gen_random_bytes() server_default
  caused sqlite table creation to fail.

- Add missing back_populates relationships to stub models so SQLAlchemy
  mapper configuration succeeds. Purchase.user and Store.user_accounts
  were missing, causing "has no property" errors during Base.metadata.create_all.

- Auto-fix ruff import sorting (I001) across all source and test files.

- Manually fix line-too-long (E501) in config.py.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

tests/test_config.py

@@ -1,4 +1,5 @@
 import pytest
+
 from receiptwitness.config import ReceiptWitnessSettings