feat: implement error handling and validation middleware, enhance result handling

Co-authored-by: Copilot <copilot@github.com>

apps/backend/src/lib/http/error-handler.ts

@@ -0,0 +1,20 @@
+// http/error-handler.ts
+import type { MiddlewareHandler } from 'hono'
+import { unexpectedProblem } from './problem-builders'
+import { logger } from '../logger'
+
+export const errorHandler: MiddlewareHandler = async (c, next) => {
+  try {
+    await next()
+  } catch (error) {
+    const requestId = c.get('requestId') as string | undefined
+    const instance = requestId ? `/requests/${requestId}` : undefined
+
+    logger.error({
+      requestId,
+      error
+    })
+
+    return c.json(unexpectedProblem({ instance }), 500)
+  }
+}