feat: add deposit and weekend km allowance to vehicles, update migrations and booking flow

Co-authored-by: Copilot <copilot@github.com>

supabase/migrations/04-kaution-weekend-km.sql

@@ -0,0 +1,27 @@
+-- =============================================================================
+-- MC Cars - Add per-vehicle kaution (deposit) and max_km_weekend columns.
+-- Idempotent.
+-- =============================================================================
+
+-- -----------------------------------------------------------------------------
+-- 1. kaution_eur: deposit per vehicle, NOT NULL, default 5000, must be > 0
+-- -----------------------------------------------------------------------------
+alter table public.vehicles add column if not exists kaution_eur integer not null default 5000;
+
+do $$
+begin
+    if not exists (
+        select 1 from information_schema.check_constraints
+        where constraint_name = 'vehicles_kaution_positive'
+    ) then
+        alter table public.vehicles add constraint vehicles_kaution_positive check (kaution_eur > 0);
+    end if;
+end $$;
+
+-- -----------------------------------------------------------------------------
+-- 2. max_km_weekend: optional per-weekend-day km allowance (NULL = use max_daily_km)
+-- -----------------------------------------------------------------------------
+alter table public.vehicles add column if not exists max_km_weekend integer default null;
+
+-- Signal PostgREST to reload its schema cache
+notify pgrst, 'reload schema';