fix(security): close member self-escalation + harden after full audit

Full-application audit (5 parallel read-only audits). Two agent
"blockers" were false alarms and verified as already-correct. Real
findings fixed:

- Self-escalation (BLOCKER): migration 024 + schema.sql section 16
  extend prevent_member_self_role_change to also block a
  non-privileged member changing their own tier_id (self tier
  upgrade) and onboarding_completed_at (skipping required
  code-of-conduct via a raw PostgREST call). finishOnboarding and
  skipOnboarding now set onboarding_completed_at through the service
  client AFTER their server-side required-steps check, so the
  legitimate path is unaffected.
- Cross-tenant write scoping: added .eq('space_id', member.space_id)
  to updateForumThread, deleteForumThread, editComment, deleteComment,
  deleteChannel, renameChannel (RLS-only -> defence-in-depth).
- generateInviteCode: crypto.getRandomValues, widened to 8 chars
  (was Math.random, ~21 bits, enumerable under the join rate limit).
- SafeMarkdown: dropped iframe and the global style attribute
  (no unsandboxed embeds / UI-redress); markdown + safe HTML stay.
- /settings: server-side admin redirect before fetching space and
  integration config (was client-hidden only).
- app/(app)/layout.tsx: export const dynamic = 'force-dynamic'.
- Bounded queries: loadComments .limit(500), tasks .limit(2000).

Backlog from the audit is tracked in HANDOFF (import Zod schemas,
types/database.ts + docs regen, email lowercasing, unauthorized-
handling consistency, KB-modal ACL hookup).

271/271 tests pass. Build clean. Migrations 023 + 024 idempotent,
apply via deploy.sh on next deploy.

lib/auth-actions.ts

@@ -6,11 +6,15 @@ import { redirect } from 'next/navigation'
 import { checkRateLimit, sanitizeString, sanitizeSlug } from '@/lib/security'
 
 function generateInviteCode() {
-  const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
+  // Cryptographically random, and wider (8 chars) so the code space is not
+  // brute-forceable under the joinSpace rate limit.
+  const chars = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789'
   const prefix = 'HSL'
   const year = new Date().getFullYear()
+  const bytes = new Uint8Array(8)
+  crypto.getRandomValues(bytes)
   let code = ''
-  for (let i = 0; i < 4; i++) code += chars.charAt(Math.floor(Math.random() * chars.length))
+  for (let i = 0; i < 8; i++) code += chars[bytes[i] % chars.length]
   return `${prefix}-${year}-${code}`
 }