feat: relate Request.employee_id to Employee via FK

Retrofits Request.employeeId from a free-text String with no
referential integrity to a real relation: employeeId String @db.Uuid
+ employee Employee @relation(...), plus the inverse Employee.requests
back-relation field. Migration alters requests.employee_id from TEXT
to UUID with an FK constraint (drop+recreate, confirmed empty table -
no production/dev data existed to migrate).

prisma/migrations/20260731075828_relate_request_to_employee/migration.sql

@@ -0,0 +1,12 @@
+/*
+  Warnings:
+
+  - Changed the type of `employee_id` on the `requests` table. No cast exists, the column would be dropped and recreated, which cannot be done if there is data, since the column is required.
+
+*/
+-- AlterTable
+ALTER TABLE "requests" DROP COLUMN "employee_id",
+ADD COLUMN     "employee_id" UUID NOT NULL;
+
+-- AddForeignKey
+ALTER TABLE "requests" ADD CONSTRAINT "requests_employee_id_fkey" FOREIGN KEY ("employee_id") REFERENCES "employees"("id") ON DELETE RESTRICT ON UPDATE CASCADE;