feat(transport): support multi-leg (layover) flights in the booking form

A flight booking can now hold an ordered chain of airports (e.g. FRA -> BER ->
HND) instead of a single departure/arrival pair. The route is entered as a list
of waypoints with a '+ add stop' button; each stop carries its own arrival and
departure time plus the airline/flight number of the segment leaving it, while
the whole booking keeps one price.

Stored without a schema change: the existing reservation_endpoints rows carry the
ordered waypoints (from/stop/to by sequence) and a metadata.legs array holds the
per-leg detail. Top-level metadata (departure_airport/arrival_airport/airline/
flight_number) mirrors the first and last leg, so a single-leg flight persists
exactly as before and legacy readers keep working.

client/src/utils/flightLegs.ts

@@ -0,0 +1,105 @@
+// Multi-leg (layover) flight support.
+//
+// A flight booking is ONE reservation whose route is an ordered chain of airports
+// (e.g. FRA -> BER -> HND). The geometry + order are the source of truth in
+// `reservation.endpoints` (role 'from' for the first airport, 'stop' for each
+// intermediate one, 'to' for the last, ordered by `sequence`). The per-leg detail
+// — airline, flight number, and each segment's own day/time — lives in
+// `metadata.legs`. The top-level metadata (`departure_airport`/`arrival_airport`/
+// `airline`/`flight_number`) and `day_id`/`end_day_id` mirror the FIRST and LAST
+// leg so legacy readers keep working.
+//
+// A legacy single-leg flight (two endpoints, flat metadata, no `metadata.legs`)
+// is normalised here into a one-leg chain, so every renderer can use one path.
+
+import type { Reservation, ReservationEndpoint } from '../types'
+
+export interface FlightLeg {
+  from: string | null // IATA code (or null)
+  to: string | null
+  airline?: string
+  flight_number?: string
+  dep_day_id?: number | null
+  dep_time?: string | null // 'HH:mm'
+  arr_day_id?: number | null
+  arr_time?: string | null
+}
+
+/** reservation.metadata may be a JSON string or an already-parsed object. */
+export function parseReservationMetadata(r: Pick<Reservation, 'metadata'>): Record<string, any> {
+  const m = r.metadata
+  if (!m) return {}
+  if (typeof m === 'string') {
+    try {
+      let parsed = JSON.parse(m || '{}')
+      // Defensive: an earlier bug could double-encode metadata (a JSON string of a
+      // JSON string) — unwrap it once more so saved flights heal on read.
+      if (typeof parsed === 'string') { try { parsed = JSON.parse(parsed) } catch { /* keep */ } }
+      return (parsed && typeof parsed === 'object') ? parsed : {}
+    } catch { return {} }
+  }
+  return m as Record<string, any>
+}
+
+/** Endpoints ordered by `sequence` (geometry + order source of truth). */
+export function orderedEndpoints(r: Pick<Reservation, 'endpoints'>): ReservationEndpoint[] {
+  return (r.endpoints || []).slice().sort((a, b) => (a.sequence ?? 0) - (b.sequence ?? 0))
+}
+
+/**
+ * Ordered legs of a flight. `metadata.legs` is preferred; otherwise a single leg
+ * is derived from the endpoints (and finally the flat metadata) so that legacy
+ * single-leg flights — and flights created before this feature — still work.
+ */
+export function getFlightLegs(r: Reservation): FlightLeg[] {
+  const meta = parseReservationMetadata(r)
+  if (Array.isArray(meta.legs) && meta.legs.length > 0) {
+    return meta.legs.map((l: any): FlightLeg => ({
+      from: l.from ?? null,
+      to: l.to ?? null,
+      airline: l.airline || undefined,
+      flight_number: l.flight_number || undefined,
+      dep_day_id: l.dep_day_id ?? null,
+      dep_time: l.dep_time ?? null,
+      arr_day_id: l.arr_day_id ?? null,
+      arr_time: l.arr_time ?? null,
+    }))
+  }
+  // Legacy fallback: one leg from the endpoints / flat metadata.
+  const eps = orderedEndpoints(r)
+  const first = eps[0]
+  const last = eps[eps.length - 1]
+  const fromCode = first?.code ?? meta.departure_airport ?? null
+  const toCode = last?.code ?? meta.arrival_airport ?? null
+  if (!fromCode && !toCode) return []
+  return [{
+    from: fromCode,
+    to: toCode,
+    airline: meta.airline || undefined,
+    flight_number: meta.flight_number || undefined,
+    dep_day_id: r.day_id ?? null,
+    dep_time: first?.local_time ?? null,
+    arr_day_id: r.end_day_id ?? r.day_id ?? null,
+    arr_time: last?.local_time ?? null,
+  }]
+}
+
+/** Number of flight segments. 1 for a simple from -> to booking. */
+export function legCount(r: Reservation): number {
+  return getFlightLegs(r).length
+}
+
+export function isMultiLegFlight(r: Reservation): boolean {
+  return r.type === 'flight' && legCount(r) > 1
+}
+
+/**
+ * Ordered route labels (IATA codes, or names when no code) for display, e.g.
+ * ['FRA','BER','HND']. Uses endpoints; falls back to the flat metadata pair.
+ */
+export function routeStops(r: Reservation): string[] {
+  const eps = orderedEndpoints(r)
+  if (eps.length >= 2) return eps.map(e => e.code || e.name)
+  const meta = parseReservationMetadata(r)
+  return [meta.departure_airport, meta.arrival_airport].filter(Boolean) as string[]
+}