fix(invites): close single-use invite max_uses TOCTOU (H1)

Redeeming an invite read uses_count, checked the cap in JS, then did a
non-atomic uses_count = uses_count + 1 write AFTER creating the membership.
Two concurrent redemptions of a max_uses=1 invite both passed the check and
both created a member, and the counter under-counted.

Fix: claim a use ATOMICALLY before creating the membership via a
compare-and-swap on uses_count (lib/invite-claim.ts) — the UPDATE only writes
when uses_count still equals the value the cap decision was based on, so of two
racing claims exactly one wins. The loser re-reads and retries while capacity
remains (multi-use invites unaffected). On a failed membership insert the claim
is rolled back so a slot is not burned.

Extracted to lib/invite-claim.ts (out of the 'use server' surface, unit-tested).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/dev/types/routes.d.ts";
+import "./.next/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.