Enforce requireClubMembership on social.getPost, addComment, comments,
like, unlike, challenges.leave, and challenges.submitProgress so private
club content is not readable/actionable by non-members (was IDOR).
Extract the membership helpers (requireClubMembership,
resolveClubIdFromPost, resolveClubIdFromChallenge) into a shared
dependency-free module (nessa-community-authz.ts) so all membership-gated
endpoints use one implementation and the libsql connection surface is
typed uniformly. Each post/challenge endpoint now resolves the owning
clubId first (NOT_FOUND if the resource is missing) then gates on it.
Add regression tests (nessa-community-authz.test.ts) covering: non-member
FORBIDDEN vs member allowed for all 7 endpoints' resolve→require sequences,
NOT_FOUND for missing post/challenge, and a join→allowed→leave→blocked
integration.
