tasks: add individual completion deletion API and UI

src/routes/api/tasks/completions/[id]/+server.ts

@@ -0,0 +1,22 @@
+import type { RequestHandler } from '@sveltejs/kit';
+import { TaskCompletion } from '$models/TaskCompletion';
+import { dbConnect } from '$utils/db';
+import { error, json } from '@sveltejs/kit';
+
+export const DELETE: RequestHandler = async ({ params, locals }) => {
+  const auth = await locals.auth();
+  if (!auth?.user?.nickname) throw error(401, 'Not logged in');
+
+  await dbConnect();
+
+  const completion = await TaskCompletion.findById(params.id);
+  if (!completion) throw error(404, 'Completion not found');
+
+  if (completion.completedBy !== auth.user.nickname) {
+    throw error(403, 'You can only delete your own completions');
+  }
+
+  await completion.deleteOne();
+
+  return json({ success: true });
+};