feat(api): GET /api/missions + POST /api/missions/{id}/accept

Routes call ensureSave first (mission functions assume an initialized save),
map MissionError to 404/409 and non-integer path ids to 400. OpenAPI spec in
code gains both paths + the Mission schema (12 -> 14 paths); openapi.test.ts
covers the new shapes.

app/api/missions/route.ts

@@ -0,0 +1,10 @@
+import { getGameDb } from '@/lib/db/client'
+import { ensureSave } from '@/lib/game/save'
+import { getMissions } from '@/lib/game/missions'
+
+// The full mission board for the single active save; auto-initializes on first hit.
+export async function GET() {
+  const db = await getGameDb()
+  await ensureSave(db) // top-up inside getMissions assumes a save exists
+  return Response.json(await getMissions(db))
+}