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;

prisma/schema.prisma

@@ -27,7 +27,13 @@ enum RequestType {
 
 model Request {
   id         String      @id @default(uuid()) @db.Uuid
-  employeeId String      @map("employee_id")
+  // Was a free-text String with no referential integrity — retrofitted to a
+  // real FK once Employee records existed to point to (see Employee model
+  // above). No production data existed at the time of this change (`requests`
+  // table was empty), so this was a plain column-type migration, not a
+  // backfill.
+  employeeId String      @map("employee_id") @db.Uuid
+  employee   Employee    @relation(fields: [employeeId], references: [id])
   type       RequestType
   reason     String
   // Only the 'Draft' state is implemented so far. Deliberately kept as a
@@ -51,6 +57,7 @@ model Employee {
   managerId  String?    @map("manager_id") @db.Uuid
   manager    Employee?  @relation("EmployeeManager", fields: [managerId], references: [id])
   reports    Employee[] @relation("EmployeeManager")
+  requests   Request[]
   createdAt  DateTime   @default(now()) @map("created_at") @db.Timestamptz(6)
 
   @@map("employees")