feat: add project collaborators schema

Introduced `projectCollaborators` schema to define project-user relationships. Includes indices and a unique constraint for `projectId` and `userId`. Added corresponding type definitions for select and insert operations.

backend/src/database/schema/projectCollaborators.ts

@@ -0,0 +1,25 @@
+import { pgTable, uuid, timestamp, index, uniqueIndex } from 'drizzle-orm/pg-core';
+import { projects } from './projects';
+import { users } from './users';
+
+/**
+ * Project Collaborators relation table schema
+ */
+export const projectCollaborators = pgTable('project_collaborators', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  projectId: uuid('projectId').notNull().references(() => projects.id, { onDelete: 'cascade' }),
+  userId: uuid('userId').notNull().references(() => users.id, { onDelete: 'cascade' }),
+  createdAt: timestamp('createdAt', { withTimezone: true }).defaultNow().notNull()
+}, (table) => {
+  return {
+    projectIdIdx: index('pc_projectId_idx').on(table.projectId),
+    userIdIdx: index('pc_userId_idx').on(table.userId),
+    projectUserUniqueIdx: uniqueIndex('pc_project_user_unique_idx').on(table.projectId, table.userId)
+  };
+});
+
+/**
+ * ProjectCollaborators type definitions
+ */
+export type ProjectCollaborator = typeof projectCollaborators.$inferSelect;
+export type NewProjectCollaborator = typeof projectCollaborators.$inferInsert;