feat: migrate receiptwitness to standalone repo with inlined common

Migrates receiptwitness to a standalone repo with inlined common models. Includes SQLite test compatibility fixes (server_default stripping for gen_random_bytes, relationship stubs for SQLAlchemy mapper).

QA PASS (cartsnitch-qa): lint ✅ test ✅ 306/306 passed.

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

tests/test_pipeline/conftest.py

@@ -1,16 +1,31 @@
 """Shared test fixtures for pipeline tests."""
 
+import secrets
+
 import pytest
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, event
 from sqlalchemy.orm import sessionmaker
 
 from receiptwitness.shared.models import Base
+from receiptwitness.shared.models.user import User
+
+
+@event.listens_for(User, "before_insert")
+def _populate_email_inbound_token(mapper, connection, target):
+    """Populate email_inbound_token with a secure random value when unset.
+
+    SQLite has no gen_random_bytes() function, so we generate it in Python
+    instead of relying on the PostgreSQL server_default.
+    """
+    if target.email_inbound_token is None:
+        target.email_inbound_token = secrets.token_urlsafe(16)
 
 
 @pytest.fixture
 def engine():
     """In-memory SQLite engine for unit tests."""
     eng = create_engine("sqlite:///:memory:")
+    User.__table__.c.email_inbound_token.server_default = None
     Base.metadata.create_all(eng)
     yield eng
     eng.dispose()