feat(logistics): departure-after-arrival validation + travel/visa CSV export

- upsertFlightDetail throws BAD_REQUEST when departureAt < arrivalAt
- Travel tab: Download CSV button (project/attendee/email/flight fields/status/visa)
- Visas tab: Download CSV button (project/attendee/nationality/status/dates/notes)
- TDD: 2 new tests (rejects invalid, accepts valid); all 6 flight tests pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/server/routers/logistics.ts

@@ -162,6 +162,12 @@ export const logisticsRouter = router({
       for (const [k, v] of Object.entries(rest)) {
         if (v !== undefined) data[k] = v
       }
+      if (input.arrivalAt && input.departureAt && input.departureAt < input.arrivalAt) {
+        throw new TRPCError({
+          code: 'BAD_REQUEST',
+          message: 'Departure must be after arrival',
+        })
+      }
       const detail = await ctx.prisma.flightDetail.upsert({
         where: { attendingMemberId },
         create: { attendingMemberId, ...(data as object) },