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/src/lib/avatar.ts

@@ -0,0 +1,17 @@
+import { createHash } from 'node:crypto';
+
+function getGravatarUrl(email: string): string {
+  const hash = createHash('md5').update(email.trim().toLowerCase()).digest('hex');
+  return `https://www.gravatar.com/avatar/${hash}?d=identicon`;
+}
+
+export function resolveAvatarUrl(input: {
+  email: string;
+  image?: string | null;
+}): string {
+  if (input.image) {
+    return input.image;
+  }
+
+  return getGravatarUrl(input.email);
+}