feat(api): POST /api/turn ticks the queue one turn; an empty queue is a free skip (#44)

app/api/turn/route.ts

@@ -0,0 +1,21 @@
+import { getGameDb } from '@/lib/db/client'
+import { getWorldState, runTurn } from '@/lib/game/save'
+
+// The manual tick (spec §3): advance exactly one turn — every ACTIVE task ages by one, tasks
+// that reach 0 resolve (the 4a resolvers + their mission hooks, 4b), and PENDING successors
+// auto-start in FIFO order. An empty queue is a FREE SKIP: turn +1 with nothing else changed —
+// the route never rejects a tick, so the UI's NEXT TURN button is never disabled. No request
+// body. Returns the same world snapshot GET /api/world returns, composed with the tick's
+// task array (the snapshot readers do not select the tasks table, so it comes from the TickResult).
+export async function POST() {
+  try {
+    const db = await getGameDb()
+    const tick = await runTurn(db)
+    return Response.json({ ...(await getWorldState(db)), tasks: tick.tasks })
+  } catch (err) {
+    // No domain errors escape runTurn (4b fails the task instead) — anything reaching here is
+    // a non-domain rethrow, i.e. a real bug: the error-mapping pattern, 500 when no status.
+    const e = err as { status?: number; message: string }
+    return Response.json({ error: e.message }, { status: e.status ?? 500 })
+  }
+}