docs: align courier.com references with app.courier.com + add Mike upgrade guide

The Mike->Courier rebrand left a few courier.com references pointing at an
unrelated freight company's registered domain. Realign them to
app.courier.com (the actual deployment, matching frontend metadataBase):

- README.md (banner image + website bullet)
- frontend/src/app/signup/page.tsx (terms + privacy links)
- frontend/src/app/support/page.tsx (URL placeholder)
- frontend/src/app/components/site-logo.tsx (production landing href)

Also adds an 'Upgrading from pre-rebrand Mike deploys' section to README
that documents the two operational breaking changes introduced by the
rebrand so existing operators know what to do at deploy time:

- Encryption salt rotation (user_api_keys + user_mcp_connectors rows
  encrypted with the old mike-*-v1 salts are unreadable across the
  courier-*-v1 salt flip; provides a SQL wipe path and a one-off
  re-encryption script path).
- R2 bucket rename (the default R2_BUCKET_NAME flipped from mike to
  courier; provides aws s3 sync + rollback commands).