feat(portal-url): accept return_to param for stale-tab recovery

[rrader26]

Lets the host app pass ?return_to= when minting a customer portal URL. The generated URL embeds it as a query param so lago-front (separate PR) can bounce the user back when the JWT expires.

Behavior

• GET /customers/:id/portal_url?return_to=https://app.example.com/billing returns: https://lago-front/customer-portal/?return_to=https%3A%2F%2Fapp.example.com%2Fbilling
• Without return_to: same URL shape as today (backward compatible)

Companion PRs

• ThinkfleetAI/lago-front PR feat(portal-url): accept return_to param for stale-tab recovery #2: Apollo error handler reads return_to and redirects on expired_jwt_token

Test plan

• GET /customers/:id/portal_url?return_to=https://example.com → URL includes ?return_to=https%3A%2F%2Fexample.com
• GET /customers/:id/portal_url (no param) → URL has no query string
• Special chars in return_to are properly CGI.escape'd