test: regression coverage for error mapping and client error extraction

- handlers: pgx.ErrNoRows→404, InputError→400 with message preserved,
  pgconn 23505→409 without constraint/SQLSTATE leak, unknown→500
- services: export NewInputError so error construction is testable
- frontend: vitest for errorMessage helper — server reason preferred,
  status text and fallbacks verified

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/src/lib/__tests__/errorMessage.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+
+import { errorMessage } from "../api";
+
+describe("errorMessage", () => {
+  it("prefers the server's reason over the HTTP status text", () => {
+    const apiError = {
+      message: "Bad Request",
+      body: { error: { message: "webhook url must be https (http allowed only for private/self-hosted targets)" } },
+    };
+    expect(errorMessage(apiError)).toBe(
+      "webhook url must be https (http allowed only for private/self-hosted targets)",
+    );
+  });
+
+  it("falls back to error.message when no body reason exists", () => {
+    expect(errorMessage(new Error("network down"))).toBe("network down");
+  });
+
+  it("uses the fallback for non-error values", () => {
+    expect(errorMessage(null, "Nope")).toBe("Nope");
+    expect(errorMessage(undefined)).toBe("Request failed");
+    expect(errorMessage("string failure")).toBe("Request failed");
+  });
+
+  it("ignores empty body and message strings", () => {
+    expect(errorMessage({ message: "", body: { error: { message: "" } } }, "Empty")).toBe("Empty");
+  });
+});