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_pipeline/test_normalization.py

@@ -3,8 +3,6 @@
 import uuid
 from datetime import UTC, datetime
 
-from receiptwitness.shared.models import NormalizedProduct
-
 from receiptwitness.pipeline.normalization import (
     MatchMethod,
     clean_name,
@@ -14,6 +12,7 @@ from receiptwitness.pipeline.normalization import (
     match_by_upc,
     normalize_product,
 )
+from receiptwitness.shared.models import NormalizedProduct
 
 
 class TestCleanName: