fix: make quota check+increment atomic in license route

[PriyeshPandey2000]

Problem

frontend/app/api/license/check/route.ts reads pagesUsed with kv.get then increments with kv.incrby in two separate operations. Concurrent uploads can both pass the limit check before either increments, allowing over-consumption of the page quota.

const used = (await kv.get<number>(key)) ?? 0   // read
if (used + pages > limit) { ... deny ... }
await kv.incrby(key, pages)                      // write — not atomic with read

Fix

Use a single atomic INCRBY then check if the result exceeds the limit. Roll back if over:

const newUsed = await kv.incrby(key, pages)
if (newUsed > limit) {
  await kv.incrby(key, -pages) // undo
  return deny(newUsed - pages, limit)
}
return allow(newUsed, limit)

Or use a Lua script for true atomicity (single round-trip, no undo needed).

Priority

Low — only matters under concurrent uploads from the same token. Not an issue at current usage levels.