fix: race condition hardening across auth, counters, and data fetching (#102)

* fix: prevent race conditions in token operations and auth flows

Backend:
- Refresh token rotation: use atomic UPDATE...WHERE revoked_at IS NULL
  to prevent concurrent refresh requests from both succeeding
- Account invite codes: SELECT FOR UPDATE to prevent double-spend
- Platform invite codes: SELECT FOR UPDATE to prevent double-spend
- Password reset tokens: SELECT FOR UPDATE to prevent double-use
- Email verification tokens: SELECT FOR UPDATE to prevent double-use

Frontend:
- Token refresh subscriber arrays: swap before iterating so a throwing
  callback doesn't leave the queue in a dirty state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: atomic counters, plan limit re-check, and double-submit guard

Backend:
- Tree usage_count: use SQL-level UPDATE (Tree.usage_count + 1) instead
  of Python-level increment to prevent lost updates under concurrency
- Tag usage_count: same SQL-level atomic increment/decrement in both
  create_tree and update_tree (delete_tree already used this pattern)
- Plan tree limit: re-check count after db.flush() to close the TOCTOU
  window where two concurrent creates could both pass the pre-check

Frontend:
- TreeEditorPage: add isSaving early-return guard inside handleSaveDraft
  and handlePublish callbacks so Ctrl+S can't bypass the button disabled
  prop and fire duplicate save requests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: prevent stale API responses from overwriting newer data

- SessionHistoryPage: move loadSessions into effect with cancelled flag
  so rapid filter/tab changes discard outdated responses
- TreeLibraryPage: add request ID ref to loadTrees so stale responses
  from previous filter selections are discarded
- QuickStartPage: add request ID ref to debounced search so out-of-order
  responses don't overwrite newer search results

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add flexible intake design — deferred variables + prepared sessions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -75,15 +75,19 @@ let refreshSubscribers: ((token: string) => void)[] = []
 let refreshFailSubscribers: ((error: unknown) => void)[] = []
 
 function onRefreshed(token: string) {
-  refreshSubscribers.forEach(cb => cb(token))
+  // Swap arrays before iterating — if a callback throws, the arrays
+  // are already cleared so the next refresh cycle starts clean.
+  const subscribers = refreshSubscribers
   refreshSubscribers = []
   refreshFailSubscribers = []
+  subscribers.forEach(cb => cb(token))
 }
 
 function onRefreshFailed(error: unknown) {
-  refreshFailSubscribers.forEach(cb => cb(error))
+  const failSubscribers = refreshFailSubscribers
   refreshSubscribers = []
   refreshFailSubscribers = []
+  failSubscribers.forEach(cb => cb(error))
 }
 
 // Response interceptor - handle token refresh