Session gaps: per-session waitlist, door session picker, all-sessions choice

- Waitlist per session: entries for pick-sessions tickets name a session;
  offers use that session's free places, never run past its start, and
  claim links tick it. Held offers count towards session capacity.
- At the door: the Select items pop-up has the session picker, with
  today's session preselected for new attendees.
- On pick-sessions events every ticket shows the session list straight
  away. "All sessions" tickets record the ticked sessions
  (RegistrationOption.sessionIds) at one price; scanning, attendance,
  certificates, reminders and capacity follow them.
- Fix: per-session stock limits weren't enforced (takenCount filtered
  WaitlistEntry by a sessionId column that didn't exist, and the error
  was swallowed), and the public event page lost its stock info.
- Fix: editing a door registration with picked sessions couldn't be saved.
- Fix: the offline-scanning service worker served /_next/static cache-first
  forever, so dev browsers that had opened the scanner ran stale code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/prisma/migrations/20261023083152_add_waitlist_session/migration.sql

@@ -0,0 +1,8 @@
+-- AlterTable
+ALTER TABLE "WaitlistEntry" ADD COLUMN     "sessionId" TEXT;
+
+-- CreateIndex
+CREATE INDEX "WaitlistEntry_sessionId_idx" ON "WaitlistEntry"("sessionId");
+
+-- AddForeignKey
+ALTER TABLE "WaitlistEntry" ADD CONSTRAINT "WaitlistEntry_sessionId_fkey" FOREIGN KEY ("sessionId") REFERENCES "EventSession"("id") ON DELETE CASCADE ON UPDATE CASCADE;

backend/prisma/migrations/20261023091312_add_chosen_sessions/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "RegistrationOption" ADD COLUMN     "sessionIds" TEXT[] DEFAULT ARRAY[]::TEXT[];

backend/prisma/schema.prisma

@@ -344,6 +344,9 @@ model RegistrationOption {
   // everything else.
   sessionId      String?
   session        EventSession?               @relation(fields: [sessionId], references: [id], onDelete: Restrict)
+  // An all-sessions option on a "pick sessions" event: the sessions chosen (one line,
+  // one ticket, one price). Empty = every session.
+  sessionIds     String[]                    @default([])
 
   @@index([registrationId])
   @@index([eventOptionId])
@@ -1364,6 +1367,8 @@ model WaitlistEntry {
   eventId         String
   eventOptionId   String
   variantId       String?
+  // "Pick sessions" options: the session they're waiting for. Null for everything else.
+  sessionId       String?
   userId          String
   quantity        Int
   position        Int
@@ -1379,12 +1384,14 @@ model WaitlistEntry {
   event           Event          @relation(fields: [eventId], references: [id], onDelete: Cascade)
   eventOption     EventOption    @relation(fields: [eventOptionId], references: [id], onDelete: Cascade)
   variant         OptionVariant? @relation(fields: [variantId], references: [id], onDelete: Cascade)
+  session         EventSession?  @relation(fields: [sessionId], references: [id], onDelete: Cascade)
   user            User           @relation(fields: [userId], references: [id], onDelete: Cascade)
   registration    Registration?  @relation(fields: [registrationId], references: [id], onDelete: SetNull)
 
   @@index([eventId, status, position])
   @@index([eventOptionId, status])
   @@index([variantId])
+  @@index([sessionId])
   @@index([userId])
   @@index([registrationId])
 }
@@ -1483,6 +1490,7 @@ model EventSession {
   usages      TicketUsage[]
   reminders   SessionReminderLog[]
   registrationOptions RegistrationOption[]
+  waitlistEntries     WaitlistEntry[]
 
   @@index([eventId, startsAt])
 }