add configurable appointment slot duration per professional

DB: slot_duration_minutes INT DEFAULT 30 on professionals table
Backend DTO: slot_duration_minutes (optional int, 5-480 min) in UpdateProfessionalDto

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/prisma/migrations/0005_add_slot_duration_minutes/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE "professionals" ADD COLUMN IF NOT EXISTS "slot_duration_minutes" INTEGER NOT NULL DEFAULT 30;

backend/prisma/schema.prisma

@@ -96,6 +96,7 @@ model professionals {
   latitude               Decimal?          @db.Decimal(10, 7)
   longitude              Decimal?          @db.Decimal(10, 7)
   average_score          Decimal?          @default(0) @db.Decimal(3, 2)
+  slot_duration_minutes  Int               @default(30)
   is_active              Boolean?          @default(true)
   created_at             DateTime          @default(now()) @db.Timestamptz(6)
   updated_at             DateTime          @default(now()) @db.Timestamptz(6)