feat(booking): API réservation + availability + lib métier

Récupéré du workspace Backend (3 fichiers, 406 lignes) :
- src/lib/booking.ts : logique métier réservation
- src/app/api/bookings/route.ts : POST/GET bookings
- src/app/api/carbets/[carbetId]/availability/route.ts : calendrier dispo

Le schéma Booking/Availability était déjà dans main.

src/lib/booking.ts

@@ -0,0 +1,50 @@
+import { UserRole } from "@/generated/prisma/enums";
+
+export const DAY_MS = 24 * 60 * 60 * 1000;
+
+export function parseIsoDate(value: unknown): Date | null {
+  if (typeof value !== "string") {
+    return null;
+  }
+
+  const date = new Date(value);
+  if (Number.isNaN(date.getTime())) {
+    return null;
+  }
+
+  return date;
+}
+
+export function normalizeUtcDayStart(date: Date): Date {
+  return new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate()));
+}
+
+export function hasOverlap(startA: Date, endA: Date, startB: Date, endB: Date): boolean {
+  return startA < endB && endA > startB;
+}
+
+export function enumerateUtcDays(start: Date, end: Date): Date[] {
+  const days: Date[] = [];
+  const cursor = normalizeUtcDayStart(start);
+  const limit = normalizeUtcDayStart(end);
+
+  while (cursor < limit) {
+    days.push(new Date(cursor));
+    cursor.setUTCDate(cursor.getUTCDate() + 1);
+  }
+
+  return days;
+}
+
+export function isCeUserRole(role?: UserRole): boolean {
+  return role === UserRole.CE_MANAGER || role === UserRole.CE_MEMBER;
+}
+
+export function isWeekendUtcDay(date: Date): boolean {
+  const day = date.getUTCDay();
+  return day === 0 || day === 6;
+}
+
+export function isPublicAllowedByDefaultPolicy(day: Date): boolean {
+  return !isWeekendUtcDay(day);
+}