[Phase 4] Expose published occurrences on GET /bookings/occurrences #158

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

Purpose

The content site can list published departures. Checkout stays on the Rails app, where the lock and the signed-in guest are.

Scope

In scope

• GET /bookings/occurrences returns published rows: slug, offering name, start, end, places remaining, price, deposit, currency, details.
• Places remaining is capacity minus the same sum the lock uses (held, pending_review, confirmed).
• No authentication on this read. It exposes only published occurrences.
• Views are not part of this issue. When a later change adds booking screens, they use ui_* if Design is present and plain markup if it is not. This issue does not generate those screens.
• Do not generate a marketing site, an FAQ, or an Astro app.

Out of scope

• Holding a place through this endpoint.
• Prices in a second currency. One currency per occurrence. A second figure on a marketing page is display, in the content site.

How to deliver

Branch <issue-number>-occurrences-read. PR title [#<issue-number>] List published occurrences for the content site.

Acceptance Criteria

• A published occurrence is in the JSON. A draft one is not.
• Places remaining matches the lock's sum, including a pending_review booking.
• The endpoint does not create a booking and does not call Stripe.