Fix early-bird price blending and mislabeling; add contact-only events

- Early-bird pricing: RegistrationOption now tracks each purchase as a
  separate price tranche instead of overwriting a single price/quantity
  on repeat purchases, so buying more tickets after a tier expires no
  longer re-prices tickets already bought at the old price. Stock-limit
  checks, total-due calculation, and the Finance report's revenue-by-
  option are all tranche-aware; pages that showed one blended price per
  line now render/total each tranche. Viewing a pending/partially-paid
  registration (dashboard, detail page, or an event's registration
  list) now refreshes stale pricing on the spot instead of only at
  payment time.
- Fixed the "(early bird)" dashboard label incorrectly firing on any
  line priced below the base option price (e.g. a plain cheaper
  variant) — it now checks the real applied-tier flag.
- Added contact-only events (e.g. baptism): no registration/payment
  flow, shown on the public site with a "Contact us" popup instead of
  a Register button. Configurable via the admin event wizard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/prisma/schema.prisma

@@ -101,6 +101,10 @@ model Event {
   isActive             Boolean           @default(true)
   isHidden             Boolean           @default(false)
   requiresAuth         Boolean           @default(true)
+  requiresRegistration Boolean           @default(true)
+  contactName          String?
+  contactPhone         String?
+  contactEmail         String?
   createdAt            DateTime          @default(now())
   updatedAt            DateTime          @updatedAt
   createdById          String?
@@ -163,6 +167,7 @@ model EarlyBirdTier {
   eventOption         EventOption          @relation(fields: [eventOptionId], references: [id], onDelete: Cascade)
   variant             OptionVariant?       @relation(fields: [variantId], references: [id], onDelete: Cascade)
   registrationOptions RegistrationOption[]
+  tranches            RegistrationOptionTranche[]
 
   @@index([eventOptionId, deadline])
   @@index([variantId])
@@ -217,6 +222,7 @@ model RegistrationOption {
   variant        OptionVariant? @relation(fields: [variantId], references: [id], onDelete: SetNull)
   appliedTier    EarlyBirdTier? @relation(fields: [appliedTierId], references: [id], onDelete: SetNull)
   tickets        Ticket[]
+  tranches       RegistrationOptionTranche[]
 
   @@index([registrationId])
   @@index([eventOptionId])
@@ -224,6 +230,27 @@ model RegistrationOption {
   @@index([appliedTierId])
 }
 
+// One row per purchase-at-a-price for a RegistrationOption. Never mutated after creation
+// (mirrors the Payment model's append-only pattern) — this is what lets a single ticket
+// type be bought in multiple batches at different early-bird prices without either batch's
+// price bleeding into the other. RegistrationOption.quantity/priceSnapshot/appliedTierId
+// stay in sync as an aggregate (quantity = sum of tranche quantities; priceSnapshot/appliedTierId
+// mirror the most recently added tranche) for the many call sites that only need "how many"
+// or a single display price.
+model RegistrationOptionTranche {
+  id                    String              @id @default(uuid())
+  registrationOptionId  String
+  quantity              Int
+  priceSnapshot         Float
+  appliedTierId         String?
+  createdAt             DateTime            @default(now())
+  registrationOption    RegistrationOption  @relation(fields: [registrationOptionId], references: [id], onDelete: Cascade)
+  appliedTier           EarlyBirdTier?      @relation(fields: [appliedTierId], references: [id], onDelete: SetNull)
+
+  @@index([registrationOptionId])
+  @@index([appliedTierId])
+}
+
 model Payment {
   id                String            @id @default(uuid())
   amount            Float