fix: add startup validation to ReceiptWitness config

Add Pydantic model_validator to ReceiptWitnessSettings that fails fast
if session_encryption_key is missing or a placeholder value. Conditional
validation for resend_api_key when notifications_enabled=true.

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

receiptwitness/tests/conftest.py

@@ -1,12 +1,16 @@
 """Shared test fixtures."""
 
 import json
+import os
 from pathlib import Path
 
 import pytest
 
 FIXTURES_DIR = Path(__file__).parent / "fixtures"
 
+os.environ.setdefault("RW_SESSION_ENCRYPTION_KEY", "test-secret-key-for-unit-tests-only-32bytes!")
+os.environ.setdefault("RW_MAILGUN_WEBHOOK_SIGNING_KEY", "test-mailgun-signing-key")
+
 
 @pytest.fixture
 def meijer_receipt_data() -> dict: