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  so rapid filter/tab changes discard outdated responses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/pages/TreeLibraryPage.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useState, useCallback, useMemo } from 'react'
+import { useEffect, useState, useCallback, useMemo, useRef } from 'react'
 import { useNavigate, useSearchParams } from 'react-router-dom'
 import { X, RotateCcw, Play, FileUp } from 'lucide-react'
 import { PageMeta } from '@/components/common/PageMeta'
@@ -158,20 +158,11 @@ export function TreeLibraryPage() {
       .catch((err) => console.error('Failed to load categories:', err))
   }, [])
 
-  // Load trees when filters change
-  useEffect(() => {
-    loadTrees()
-  }, [selectedCategoryId, selectedTags, selectedFolderId, treeLibrarySortBy, typeFilter])
+  // Request ID ref to discard stale responses when filters change rapidly
+  const loadTreesRequestId = useRef(0)
 
-  // Load folders on mount and listen for changes
-  useEffect(() => {
-    loadFolders()
-    const handleFolderChange = () => loadFolders()
-    window.addEventListener('folder-changed', handleFolderChange)
-    return () => window.removeEventListener('folder-changed', handleFolderChange)
-  }, [loadFolders])
-
-  const loadTrees = async () => {
+  const loadTrees = useCallback(async () => {
+    const requestId = ++loadTreesRequestId.current
     setIsLoading(true)
     try {
       const treesData = await treesApi.list({
@@ -181,14 +172,29 @@ export function TreeLibraryPage() {
         folder_id: selectedFolderId || undefined,
         sort_by: treeLibrarySortBy,
       })
+      if (requestId !== loadTreesRequestId.current) return
       setTrees(treesData)
     } catch (err) {
+      if (requestId !== loadTreesRequestId.current) return
       toast.error('Failed to load flows')
       console.error(err)
     } finally {
-      setIsLoading(false)
+      if (requestId === loadTreesRequestId.current) setIsLoading(false)
     }
-  }
+  }, [selectedCategoryId, selectedTags, selectedFolderId, treeLibrarySortBy, typeFilter])
+
+  // Load trees when filters change
+  useEffect(() => {
+    loadTrees()
+  }, [loadTrees])
+
+  // Load folders on mount and listen for changes
+  useEffect(() => {
+    loadFolders()
+    const handleFolderChange = () => loadFolders()
+    window.addEventListener('folder-changed', handleFolderChange)
+    return () => window.removeEventListener('folder-changed', handleFolderChange)
+  }, [loadFolders])
 
   const handleSearch = async () => {
     if (!searchQuery.trim()) {