[Phase 3] Collect the balance from our page, not an emailed Stripe link #157

Epic: #151
Proposal: proposals/bookings_engine.md (local, git-excluded). Argument: https://github.com/Davidslv/notes/issues/178

Purpose

The balance is due later, sometimes months later. A Checkout link dies within 24 hours. The email points at our page. The session is created when the guest opens it.

Scope

In scope

• After confirm, create the balance instalment for price_cents minus the deposit. A null deposit_cents is one full instalment and does not create a balance.
• RequestDueInstalmentsJob emails a Bookings URL, not a Stripe URL. It skips rejected, cancelled, and expired.
• Opening the page publishes booking.balance_due.bookings with the same fields as booking.held.bookings, plus the customer id already stored on the booking.
• Billing reuses that customer. It does not create a second one. Session idempotency key is session- plus the balance payable_ref. Same inline exception as the deposit.
• payment.succeeded.billing for a balance instalment marks it paid. It does not use the held check and it does not refund for failing that check.
• The due job does not create the Stripe session.

Out of scope

• Saving the card. Charging off-session.
• A Stripe URL in the email.

How to deliver

Branch <issue-number>-balance-page. PR title [#<issue-number>] Collect the balance when the guest opens the page.

Acceptance Criteria

• The due email contains the Bookings URL and no checkout.stripe.com URL.
• Opening the page creates one session for that payable_ref and redirects to it.
• A second open does not create a second session.
• A succeeded balance marks the instalment paid and does not change a confirmed booking back to held or pending_review.
• A rejected booking gets no balance email and no balance session.