feat: add reason field to recall entity

[TaprootFreak]

Summary

• Adds mandatory reason field to newly created recalls using SEPA ISO 20022 reason codes
• Enum values: DUPL, TECH, FRAD, CUST, AM09, AC03, Unknown (fallback)
• DB column is nullable — existing recalls keep NULL, only new creates via CreateRecallDto are required to pass a reason
• UpdateRecallDto accepts reason as optional-but-not-null

Test plan

• Run migration 1776930994259-AddRecallReason on DEV, verify column exists as nvarchar(256) nullable
• POST /recall without reason → 400 validation error
• POST /recall with valid enum value → 201, row persisted with reason
• POST /recall with invalid enum value → 400 validation error
• PUT /recall/:id updating only reason → row updated, other fields untouched
• Verify existing recalls (reason = NULL) are still readable