fix(db): explicit TypeORM column types for nullable string/uuid columns

`string | null` / `uuid | null` columns reflect as Object and crash the
backend at metadata build (DataTypeNotSupportedError). Declare explicit
`type` on users.phone/email and cluster_allocation_logs FK id columns.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/users/entities/user.entity.ts

@@ -18,10 +18,13 @@ export class User {
   // optional contact field only — it is never used to authenticate.
   // Both are unique; Postgres treats NULLs as distinct, so accounts missing
   // either coexist freely.
-  @Column({ unique: true, nullable: true })
+  // NOTE: nullable string columns MUST declare an explicit `type` — TypeScript
+  // reflects the `string | null` union as `Object`, which TypeORM rejects with
+  // DataTypeNotSupportedError and crashes the backend at metadata build.
+  @Column({ type: 'varchar', unique: true, nullable: true })
   phone: string | null; // canonical E.164, e.g. +989121234567
 
-  @Column({ unique: true, nullable: true })
+  @Column({ type: 'varchar', unique: true, nullable: true })
   email: string | null;
 
   @Column({ default: false })