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/utils/http.ts

@@ -0,0 +1,46 @@
+import type { H3Event } from 'h3'
+
+import { getRouterParam } from 'h3'
+
+export function httpError(statusCode: number, statusMessage: string): never {
+  throw createError({
+    statusCode,
+    statusMessage
+  })
+}
+
+export function assertHttp(
+  condition: unknown,
+  statusCode: number,
+  statusMessage: string
+): asserts condition {
+  if (!condition) {
+    httpError(statusCode, statusMessage)
+  }
+}
+
+export function assertBadRequest(condition: unknown, statusMessage: string): asserts condition {
+  assertHttp(condition, 400, statusMessage)
+}
+
+export function getRequiredRouteParam(event: H3Event, name: string, label = name) {
+  const value = getRouterParam(event, name)
+
+  assertBadRequest(value, `${label} is required`)
+
+  return value
+}
+
+export function mapDatabaseError(
+  error: unknown,
+  handlers: Partial<Record<string, { statusCode: number, statusMessage: string }>>
+): never {
+  const code = (error as { code?: string } | null)?.code
+  const handler = code ? handlers[code] : undefined
+
+  if (handler) {
+    httpError(handler.statusCode, handler.statusMessage)
+  }
+
+  throw error
+}