feat(bookings): customer self-service rebooking with real slot picker

CustomerBookingView now carries tenantSlug/serviceId/staffId/
classSessionId so the manage page can query live availability; class
bookings resolve their session title as the display label. Reschedule
rejects waitlisted/completed bookings and session-bound class bookings
(400), collapsing a dead branch in the appointment path.

The manage page reschedule dialog replaces free-form date/time inputs
with a date → availability-slot picker filtered to appointment mode,
so customers can only move to a genuinely free slot with the correct
duration; 409 surfaces "slot just taken". Actions are status-gated:
reschedule for confirmed/pending appointments, cancel for live
bookings, explanatory note for class bookings.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

project.md

@@ -138,6 +138,14 @@
   - demo mode ships a seeded session with a populated roster; mutation attempts surface the demo notice instead of failing
   - verified live in demo mode: seeded a full session via the public booking API, roster listed 4 confirmed + 1 waitlisted, promote rejected with 409, `allowOverCapacity` promoted and drained the queue; browser-checked CS/EN rendering with a clean console
   - remaining: live staging validation on Neon + pgx repository path
+- `[done/partial]` Milestone 21: Customer self-service rebooking
+  - `CustomerBookingView` now carries `tenantSlug`, `serviceId`, `staffId`, `classSessionId` — everything the manage page needs to query real availability; class bookings resolve the session title as their service label instead of rendering blank
+  - `RescheduleBooking` hardened: rejects `waitlisted`/`completed` bookings and any class booking (session-bound times cannot move to arbitrary windows; switching sessions is cancel-and-rebook); the previously dead `ClassSessionID == nil` branch collapsed into a straight appointment validation
+  - manage page reschedule dialog replaced free-form date+time inputs with a real slot picker: pick a date → `GET /v1/public/tenants/{slug}/availability` (filtered by the booking's service/staff) → time buttons; the slot carries its own duration so the hardcoded 60-minute assumption is gone; 409 surfaces "slot just taken" instead of a generic failure
+  - action buttons are now status-gated: reschedule shows only for confirmed/pending appointments, cancel for any live booking, and class bookings get an explanatory note instead of a broken button
+  - `CustomerBookingView` schema added to OpenAPI and referenced from `GET /v1/public/bookings/{reference}`; the reschedule contract documents the class-booking 400 and slot-conflict 409
+  - verified live: booked an appointment via public API, rescheduled through the real manage page in a browser (slot grid → 200 → card shows new time); class booking reschedule returns 400
+  - remaining: live staging validation on Neon + pgx repository path
 
 ## Overview
 Bookra is a Czech-first booking SaaS for small local service businesses. It targets salons, massage therapists, small clinics, repair shops, and small studios/clubs that need a simple public booking flow, internal schedule management, reminders, and basic business reporting without becoming a full ERP.
@@ -388,7 +396,7 @@ Key entities:
 Shortlist of worthwhile directions once v1 stabilizes — kept deliberately small, in line with the "no feature sprawl" principle:
 
 - **Activity/audit log**: a per-tenant feed of who changed what (booking created/cancelled/promoted, settings edits). Natural fit for the notification dropdown and useful for multi-staff accountability.
-- **Customer self-service rebooking**: let customers move to a different free slot from the manage page instead of cancel-and-rebook; the reschedule path already validates targets, so this is mostly UI plus a picker.
+- **Session-to-session move for classes**: M21 covered appointment rebooking; class customers currently must cancel and book another session. A "move to another session of the same template" flow (target session picker + capacity/waitlist handling) would close the gap.
 - **Calendar integrations**: ICS feed per tenant/staff and optional Google Calendar sync; high demand for salons, moderate complexity. Start with a read-only `.ics` subscription URL — zero third-party auth needed.
 - **Recurring class schedules**: templates currently schedule sessions one at a time; a weekly recurrence rule with per-session overrides would cut setup time for studios.
 - **Booking notes intake**: structured intake fields per service (e.g. "first visit?", "injuries?") instead of the single free-text notes field — still schema-light.