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/auth/passkey/register/verify.post.ts

@@ -1,7 +1,9 @@
 import { verifyRegistrationResponse, type RegistrationResponseJSON } from '@simplewebauthn/server'
 
 import { requireAuth } from '../../../../utils/auth'
-import { createUserPasskey, getUserById, listUserPasskeys } from '../../../../utils/user-repository'
+import { assertBadRequest, httpError, mapDatabaseError } from '../../../../utils/http'
+import { createUserPasskey, listUserPasskeys } from '../../../../utils/user-repository'
+import { requireExistingUser } from '../../../../utils/users'
 import { buildPasskeyLabel, consumeRegistrationChallenge, getWebAuthnConfig } from '../../../../utils/webauthn'
 
 export default defineEventHandler(async (event) => {
@@ -10,20 +12,12 @@ export default defineEventHandler(async (event) => {
     response?: RegistrationResponseJSON
   }>(event)
 
-  if (!body.response) {
-    throw createError({
-      statusCode: 400,
-      statusMessage: 'Passkey registration response is required'
-    })
-  }
+  assertBadRequest(body.response, 'Passkey registration response is required')
 
   const expectedChallenge = await consumeRegistrationChallenge(auth.user.id)
 
   if (!expectedChallenge) {
-    throw createError({
-      statusCode: 400,
-      statusMessage: 'Passkey registration challenge expired. Try again.'
-    })
+    httpError(400, 'Passkey registration challenge expired. Try again.')
   }
 
   const config = getWebAuthnConfig(event)
@@ -35,10 +29,7 @@ export default defineEventHandler(async (event) => {
   })
 
   if (!verification.verified || !verification.registrationInfo) {
-    throw createError({
-      statusCode: 400,
-      statusMessage: 'Passkey registration could not be verified'
-    })
+    httpError(400, 'Passkey registration could not be verified')
   }
 
   try {
@@ -52,18 +43,16 @@ export default defineEventHandler(async (event) => {
       transports: body.response.response.transports || [],
       label: buildPasskeyLabel()
     })
-  } catch (error: any) {
-    if (error?.code === '23505') {
-      throw createError({
+  } catch (error) {
+    mapDatabaseError(error, {
+      '23505': {
         statusCode: 409,
         statusMessage: 'This passkey is already registered'
-      })
-    }
-
-    throw error
+      }
+    })
   }
 
-  const updatedUser = await getUserById(auth.user.id)
+  const updatedUser = await requireExistingUser(auth.user.id, 'Unable to load updated user')
   const passkeys = await listUserPasskeys(auth.user.id)
 
   return {