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  so rapid filter/tab changes discard outdated responses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/SessionHistoryPage.tsx

@@ -60,7 +60,59 @@ export function SessionHistoryPage() {
 
   // Load sessions when filters change
   useEffect(() => {
+    let cancelled = false
+
+    const loadSessions = async () => {
+      setIsLoading(true)
+      try {
+        const params: Record<string, string | boolean> = {}
+
+        // Tab filter (all/active/completed)
+        if (filter !== 'all') {
+          params.completed = filter === 'completed'
+        }
+
+        // Search/filter params
+        if (filters.ticketNumber) {
+          params.ticket_number = filters.ticketNumber
+        }
+        if (filters.clientName) {
+          params.client_name = filters.clientName
+        }
+        if (filters.treeName) {
+          params.tree_name = filters.treeName
+        }
+
+        // Date range params
+        if (filters.dateRange?.from) {
+          const fromDate = filters.dateRange.from
+          const toDate = filters.dateRange.to || filters.dateRange.from
+
+          if (filters.dateType === 'started') {
+            params.started_after = fromDate.toISOString()
+            params.started_before = toDate.toISOString()
+          } else {
+            params.completed_after = fromDate.toISOString()
+            params.completed_before = toDate.toISOString()
+          }
+        }
+
+        const sessionsData = await sessionsApi.list({ ...params, size: 51 })
+        if (cancelled) return
+        const truncated = sessionsData.length > 50
+        setHasMore(truncated)
+        setSessions(truncated ? sessionsData.slice(0, 50) : sessionsData)
+      } catch (err) {
+        if (cancelled) return
+        toast.error('Failed to load sessions')
+        console.error(err)
+      } finally {
+        if (!cancelled) setIsLoading(false)
+      }
+    }
+
     loadSessions()
+    return () => { cancelled = true }
   }, [filter, filters])
 
   // Update URL params when filters change
@@ -79,53 +131,6 @@ export function SessionHistoryPage() {
     setSearchParams(params, { replace: true })
   }, [filters, setSearchParams])
 
-  const loadSessions = async () => {
-    setIsLoading(true)
-    try {
-      const params: Record<string, string | boolean> = {}
-
-      // Tab filter (all/active/completed)
-      if (filter !== 'all') {
-        params.completed = filter === 'completed'
-      }
-
-      // Search/filter params
-      if (filters.ticketNumber) {
-        params.ticket_number = filters.ticketNumber
-      }
-      if (filters.clientName) {
-        params.client_name = filters.clientName
-      }
-      if (filters.treeName) {
-        params.tree_name = filters.treeName
-      }
-
-      // Date range params
-      if (filters.dateRange?.from) {
-        const fromDate = filters.dateRange.from
-        const toDate = filters.dateRange.to || filters.dateRange.from
-
-        if (filters.dateType === 'started') {
-          params.started_after = fromDate.toISOString()
-          params.started_before = toDate.toISOString()
-        } else {
-          params.completed_after = fromDate.toISOString()
-          params.completed_before = toDate.toISOString()
-        }
-      }
-
-      const sessionsData = await sessionsApi.list({ ...params, size: 51 })
-      const truncated = sessionsData.length > 50
-      setHasMore(truncated)
-      setSessions(truncated ? sessionsData.slice(0, 50) : sessionsData)
-    } catch (err) {
-      toast.error('Failed to load sessions')
-      console.error(err)
-    } finally {
-      setIsLoading(false)
-    }
-  }
-
   const handleFilterChange = (newFilters: SessionFilterState) => {
     setFilters(newFilters)
   }