feat: wire Google Wallet into loyalty enrollment, stamps, and points flows

Connect the fully-implemented Google Wallet service to the loyalty module:
- Create wallet class/object on customer enrollment
- Sync wallet passes on stamp and points operations
- Expose wallet URLs in storefront API responses
- Add conditional "Add to Google Wallet" buttons on dashboard and enroll-success pages
- Use platform-wide env var config (not per-merchant DB column)
- Add Google service account patterns to .gitignore
- Add LOYALTY_GOOGLE_* fields to app Settings
- Update deployment docs and add local testing guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -168,6 +168,11 @@ deployment-local/
 secrets/
 credentials/
 
+# Google Cloud service account keys
+*-service-account.json
+google-wallet-sa.json
+orion-*.json
+
 # Alembic
 # Note: Keep alembic/versions/ tracked for migrations
 # alembic/versions/*.pyc is already covered by __pycache__