Fix user email unique constraint (#8898)

## Context
Fix wrong unique index on user email as we don't want an index on
email/deletedAt but a partial on a where condition on deletedAt. This
should enforce email unicity excluding the ones that have a deletedAt

## Test
Run
```sql
SELECT email, COUNT(*) as duplicate_count
FROM core."user"
WHERE "deletedAt" IS NULL
GROUP BY email
HAVING COUNT(*) > 1
ORDER BY duplicate_count DESC;
```
to check duplicates before running the migration

packages/twenty-server/src/database/typeorm/core/migrations/1733408604468-fixUserEmailUniqueConstraint.ts

@@ -0,0 +1,23 @@
+import { MigrationInterface, QueryRunner } from 'typeorm';
+
+export class FixUserEmailUniqueConstraint1733408604468
+  implements MigrationInterface
+{
+  name = 'FixUserEmailUniqueConstraint1733408604468';
+
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(
+      `ALTER TABLE "core"."user" DROP CONSTRAINT "UQ_USER_EMAIL"`,
+    );
+    await queryRunner.query(
+      `CREATE UNIQUE INDEX "UQ_USER_EMAIL" ON "core"."user" ("email") WHERE "deletedAt" IS NULL`,
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(`DROP INDEX "core"."UQ_USER_EMAIL"`);
+    await queryRunner.query(
+      `ALTER TABLE "core"."user" ADD CONSTRAINT "UQ_USER_EMAIL" UNIQUE ("email", "deletedAt")`,
+    );
+  }
+}