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

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

apps/backend/src/lib/errors.ts

@@ -0,0 +1,58 @@
+// lib/errors.ts
+export type ValidationIssue = {
+  path: string
+  message: string
+}
+
+export type AppError =
+  | {
+      type: 'validation'
+      message: string
+      issues?: ValidationIssue[]
+    }
+  | {
+      type: 'not_found'
+      message: string
+    }
+  | {
+      type: 'conflict'
+      message: string
+    }
+  | {
+      type: 'unauthorized'
+      message: string
+    }
+  | {
+      type: 'forbidden'
+      message: string
+    }
+  | {
+      type: 'unexpected'
+      message: string
+    }
+
+export const Errors = {
+  validation(message: string, issues?: ValidationIssue[]): AppError {
+    return { type: 'validation', message, issues }
+  },
+
+  notFound(message = 'Resource not found'): AppError {
+    return { type: 'not_found', message }
+  },
+
+  conflict(message: string): AppError {
+    return { type: 'conflict', message }
+  },
+
+  unauthorized(message = 'Unauthorized'): AppError {
+    return { type: 'unauthorized', message }
+  },
+
+  forbidden(message = 'Forbidden'): AppError {
+    return { type: 'forbidden', message }
+  },
+
+  unexpected(message = 'Unexpected error'): AppError {
+    return { type: 'unexpected', message }
+  }
+}