refactor: centralize validation, error handling, and formatting logic

Extract shared auth logic and validation rules to shared/auth.ts
Introduce utility functions for HTTP errors and user input parsing
Standardize error messages and date formatting across the app

server/api/admin/users.post.ts

@@ -1,14 +1,10 @@
-import {
-  DEFAULT_USER_PASSWORD,
-  USERNAME_PATTERN,
-  isValidPhoneNumber,
-  normalizePhoneNumber,
-  type UserRole
-} from '~~/shared/auth'
+import { DEFAULT_USER_PASSWORD, type UserRole } from '~~/shared/auth'
 
-import { normalizeUsername, requireRole } from '../../utils/auth'
+import { requireRole } from '../../utils/auth'
+import { mapDatabaseError } from '../../utils/http'
 import { hashPassword } from '../../utils/password'
 import { createUser } from '../../utils/user-repository'
+import { parseCreateUserInput } from '../../utils/users'
 
 export default defineEventHandler(async (event) => {
   const auth = await requireRole(event, 'super_admin')
@@ -19,31 +15,12 @@ export default defineEventHandler(async (event) => {
     role?: UserRole
   }>(event)
 
-  const username = normalizeUsername(body.username || '')
-  const fullName = body.fullName?.trim() || ''
-  const phoneNumber = normalizePhoneNumber(body.phoneNumber || '')
-  const role = body.role === 'super_admin' ? 'super_admin' : 'staff'
-
-  if (!USERNAME_PATTERN.test(username)) {
-    throw createError({
-      statusCode: 400,
-      statusMessage: 'Username must be 3 to 32 characters using lowercase letters, numbers, dot, dash, or underscore'
-    })
-  }
-
-  if (fullName.length < 2) {
-    throw createError({
-      statusCode: 400,
-      statusMessage: 'Full name must be at least 2 characters'
-    })
-  }
-
-  if (!isValidPhoneNumber(phoneNumber)) {
-    throw createError({
-      statusCode: 400,
-      statusMessage: 'Phone number must contain 8 to 15 digits'
-    })
-  }
+  const {
+    username,
+    fullName,
+    phoneNumber,
+    role
+  } = parseCreateUserInput(body)
 
   const passwordHash = await hashPassword(DEFAULT_USER_PASSWORD)
 
@@ -61,14 +38,12 @@ export default defineEventHandler(async (event) => {
       user,
       defaultPassword: DEFAULT_USER_PASSWORD
     }
-  } catch (error: any) {
-    if (error?.code === '23505') {
-      throw createError({
+  } catch (error) {
+    mapDatabaseError(error, {
+      '23505': {
         statusCode: 409,
         statusMessage: 'Username already exists'
-      })
-    }
-
-    throw error
+      }
+    })
   }
 })