dev-api: add developer portal, OpenAPI spec, and Flatenbadet case study

- New /developers/ page with API docs, SDKs, pricing, use cases
- OpenAPI 3.0 spec for Orders, Missions, Photos, Analytics
- Case study: Glasskiosken i Flatenbadet — complete ROI analysis
- Updated /order/ with recurring missions and frequency dropdown

services/frilans-payout/node_modules/assertion-error/index.d.ts

@@ -0,0 +1,11 @@
+type AssertionError<T = {}> = Error & T & {
+    showDiff: boolean;
+};
+
+interface AssertionErrorConstructor {
+    new<T = {}>(message: string, props?: T, ssf?: Function): AssertionError<T>;
+}
+
+declare const AssertionError: AssertionErrorConstructor;
+
+export = AssertionError;