Implement per-user post permissions and move stats into dedicated pages

lib/users.ts

@@ -0,0 +1,38 @@
+import { ObjectId } from "mongodb";
+import { getDb } from "@/lib/mongo";
+
+export const DEFAULT_DAILY_POST_LIMIT = 10;
+const SHANGHAI_OFFSET_HOURS = 8;
+
+export function getEffectiveDailyPostLimit(user?: { dailyPostLimit?: number | null }) {
+  if (typeof user?.dailyPostLimit === "number" && user.dailyPostLimit >= 0) {
+    return user.dailyPostLimit;
+  }
+
+  return DEFAULT_DAILY_POST_LIMIT;
+}
+
+export function getShanghaiDayRange(now = new Date()) {
+  const shifted = new Date(now.getTime() + SHANGHAI_OFFSET_HOURS * 60 * 60 * 1000);
+  const year = shifted.getUTCFullYear();
+  const month = shifted.getUTCMonth();
+  const day = shifted.getUTCDate();
+  const start = new Date(Date.UTC(year, month, day, -SHANGHAI_OFFSET_HOURS, 0, 0, 0));
+  const end = new Date(Date.UTC(year, month, day + 1, -SHANGHAI_OFFSET_HOURS, 0, 0, 0));
+
+  return {
+    start,
+    end,
+    startIso: start.toISOString(),
+    endIso: end.toISOString()
+  };
+}
+
+export async function findUserById(uid?: string | null) {
+  if (!uid || !ObjectId.isValid(uid)) {
+    return null;
+  }
+
+  const db = await getDb();
+  return db.collection("users").findOne({ _id: new ObjectId(uid) });
+}