feat: add complex user invitation and management features

- Implemented complex member and invitation schemas using Zod for validation.
- Created new API endpoints for inviting, accepting, revoking, and canceling complex user invitations.
- Developed handlers for managing complex users, including listing, inviting, revoking, and canceling invitations.
- Added frontend components for displaying and managing complex users and invitations.
- Introduced session storage management for pending invitations.
- Integrated Gravatar for user avatars based on email.
- Created database migration for complex invitations table.

apps/backend/prisma/complex.prisma

@@ -12,6 +12,7 @@ model Complex {
   updatedAt       DateTime      @updatedAt @map("updated_at")
   plan            Plan?         @relation(fields: [planCode], references: [code])
   users           ComplexUser[]
+  invitations     ComplexInvitation[]
   courts          Court[]
 
   @@index([planCode])
@@ -36,3 +37,20 @@ model ComplexUser {
   @@index([userId])
   @@map("complex_users")
 }
+
+model ComplexInvitation {
+  id         String   @id @db.Uuid
+  complexId   String   @map("complex_id") @db.Uuid
+  email       String   @db.VarChar(255)
+  tokenHash   String   @unique @map("token_hash")
+  expiresAt   DateTime @map("expires_at")
+  acceptedAt  DateTime? @map("accepted_at")
+  revokedAt   DateTime? @map("revoked_at")
+  createdAt   DateTime @default(now()) @map("created_at")
+  updatedAt   DateTime @updatedAt @map("updated_at")
+  complex     Complex  @relation(fields: [complexId], references: [id], onDelete: Cascade)
+
+  @@index([complexId, email])
+  @@index([email])
+  @@map("complex_invitations")
+}