feat(server): add error handling middleware

Add global error handling middleware for Hono with:
- AppError class for application-specific errors with status codes
- Errors factory for common HTTP errors (badRequest, unauthorized, etc.)
- Consistent JSON error response format
- Tests covering all error types

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

4 files changed, 272 insertions, 0 deletions

diff --git a/pkgs/server/src/index.ts b/pkgs/server/src/index.ts
index 6c89217..54c1059 100644
--- a/pkgs/server/src/index.ts
+++ b/pkgs/server/src/index.ts
@@ -1,8 +1,11 @@
 import { serve } from "@hono/node-server";
 import { Hono } from "hono";
+import { errorHandler } from "./middleware/error-handler";
 
 const app = new Hono();
 
+app.onError(errorHandler);
+
 app.get("/", (c) => {
 	return c.json({ message: "Kioku API" });
 });
diff --git a/pkgs/server/src/middleware/error-handler.test.ts b/pkgs/server/src/middleware/error-handler.test.ts
new file mode 100644
index 0000000..21d6fc1
--- /dev/null
+++ b/pkgs/server/src/middleware/error-handler.test.ts
@@ -0,0 +1,173 @@
+import { Hono } from "hono";
+import { describe, expect, it } from "vitest";
+import { AppError, Errors, errorHandler } from "./error-handler";
+
+function createTestApp() {
+	const app = new Hono();
+	app.onError(errorHandler);
+	return app;
+}
+
+describe("errorHandler", () => {
+	describe("AppError handling", () => {
+		it("returns correct status and message for AppError", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw new AppError("Custom error", 400, "CUSTOM_ERROR");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(400);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Custom error",
+					code: "CUSTOM_ERROR",
+				},
+			});
+		});
+
+		it("uses default values for AppError", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw new AppError("Something went wrong");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(500);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Something went wrong",
+					code: "INTERNAL_ERROR",
+				},
+			});
+		});
+	});
+
+	describe("Errors factory functions", () => {
+		it("handles badRequest error", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.badRequest("Invalid input");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(400);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Invalid input",
+					code: "BAD_REQUEST",
+				},
+			});
+		});
+
+		it("handles unauthorized error", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.unauthorized();
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(401);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Unauthorized",
+					code: "UNAUTHORIZED",
+				},
+			});
+		});
+
+		it("handles forbidden error", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.forbidden("Access denied");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(403);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Access denied",
+					code: "FORBIDDEN",
+				},
+			});
+		});
+
+		it("handles notFound error", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.notFound("Resource not found");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(404);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Resource not found",
+					code: "NOT_FOUND",
+				},
+			});
+		});
+
+		it("handles conflict error", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.conflict("Already exists");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(409);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Already exists",
+					code: "CONFLICT",
+				},
+			});
+		});
+
+		it("handles validationError", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.validationError("Invalid data");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(422);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Invalid data",
+					code: "VALIDATION_ERROR",
+				},
+			});
+		});
+
+		it("handles internal error", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw Errors.internal("Database connection failed");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(500);
+			expect(await res.json()).toEqual({
+				error: {
+					message: "Database connection failed",
+					code: "INTERNAL_ERROR",
+				},
+			});
+		});
+	});
+
+	describe("unknown error handling", () => {
+		it("handles generic Error with 500 status", async () => {
+			const app = createTestApp();
+			app.get("/test", () => {
+				throw new Error("Unexpected error");
+			});
+
+			const res = await app.request("/test");
+			expect(res.status).toBe(500);
+			const body = (await res.json()) as { error: { code: string } };
+			expect(body.error.code).toBe("INTERNAL_ERROR");
+		});
+	});
+});
diff --git a/pkgs/server/src/middleware/error-handler.ts b/pkgs/server/src/middleware/error-handler.ts
new file mode 100644
index 0000000..7b92940
--- /dev/null
+++ b/pkgs/server/src/middleware/error-handler.ts
@@ -0,0 +1,95 @@
+import type { Context } from "hono";
+import { HTTPException } from "hono/http-exception";
+import type { ContentfulStatusCode } from "hono/utils/http-status";
+
+/**
+ * Application-specific error with status code and optional details
+ */
+export class AppError extends Error {
+	readonly statusCode: ContentfulStatusCode;
+	readonly code: string;
+
+	constructor(
+		message: string,
+		statusCode: ContentfulStatusCode = 500,
+		code = "INTERNAL_ERROR",
+	) {
+		super(message);
+		this.name = "AppError";
+		this.statusCode = statusCode;
+		this.code = code;
+	}
+}
+
+/**
+ * Common error factory functions
+ */
+export const Errors = {
+	badRequest: (message = "Bad request", code = "BAD_REQUEST") =>
+		new AppError(message, 400, code),
+
+	unauthorized: (message = "Unauthorized", code = "UNAUTHORIZED") =>
+		new AppError(message, 401, code),
+
+	forbidden: (message = "Forbidden", code = "FORBIDDEN") =>
+		new AppError(message, 403, code),
+
+	notFound: (message = "Not found", code = "NOT_FOUND") =>
+		new AppError(message, 404, code),
+
+	conflict: (message = "Conflict", code = "CONFLICT") =>
+		new AppError(message, 409, code),
+
+	validationError: (message = "Validation failed", code = "VALIDATION_ERROR") =>
+		new AppError(message, 422, code),
+
+	internal: (message = "Internal server error", code = "INTERNAL_ERROR") =>
+		new AppError(message, 500, code),
+};
+
+interface ErrorResponse {
+	error: {
+		message: string;
+		code: string;
+	};
+}
+
+/**
+ * Global error handler middleware for Hono
+ */
+export function errorHandler(err: Error, c: Context): Response {
+	// Handle AppError
+	if (err instanceof AppError) {
+		const response: ErrorResponse = {
+			error: {
+				message: err.message,
+				code: err.code,
+			},
+		};
+		return c.json(response, err.statusCode);
+	}
+
+	// Handle Hono's HTTPException
+	if (err instanceof HTTPException) {
+		const response: ErrorResponse = {
+			error: {
+				message: err.message,
+				code: "HTTP_ERROR",
+			},
+		};
+		return c.json(response, err.status as ContentfulStatusCode);
+	}
+
+	// Handle unknown errors
+	console.error("Unhandled error:", err);
+	const response: ErrorResponse = {
+		error: {
+			message:
+				process.env.NODE_ENV === "production"
+					? "Internal server error"
+					: err.message,
+			code: "INTERNAL_ERROR",
+		},
+	};
+	return c.json(response, 500);
+}
diff --git a/pkgs/server/src/middleware/index.ts b/pkgs/server/src/middleware/index.ts
new file mode 100644
index 0000000..bd3d4dc
--- /dev/null
+++ b/pkgs/server/src/middleware/index.ts
@@ -0,0 +1 @@
+export { AppError, Errors, errorHandler } from "./error-handler";