feat: AI-assisted flow builder with 4-stage wizard

Implements the complete AI flow builder feature using a guided 4-stage
wizard (Foundation → Scaffold → Branch Detail → Review & Assemble).
AI assists at bounded points using Claude Haiku for cost-efficient
structured JSON generation (~$0.01-0.03/flow).

Backend: new models (ai_conversations, ai_usage), Alembic migration,
quota enforcement with billing anchor, Anthropic API integration with
prompt caching, tree validation, conversation CRUD with 24h TTL,
APScheduler cleanup job, 5 API endpoints, Pydantic schemas.

Frontend: TypeScript types, API client, Zustand store for wizard state,
7 components (modal, step indicator, foundation form, branch selector,
branch detail view, tree preview, quota display), MyTreesPage integration
with "Build with AI" button (hidden when AI not configured).

Tests: 14 validator unit tests + 11 endpoint integration tests with
mocked Anthropic (zero real API spend). All 25 tests passing.

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

frontend/src/api/aiBuilder.ts

@@ -0,0 +1,61 @@
+import { apiClient } from './client'
+import type {
+  AIQuotaStatus,
+  AIStartResponse,
+  AIScaffoldResponse,
+  AIBranchDetailResponse,
+  AIAssembleResponse,
+} from '@/types'
+
+export const aiBuilderApi = {
+  getQuota: async (): Promise<AIQuotaStatus> => {
+    const { data } = await apiClient.get('/ai/quota')
+    return data
+  },
+
+  start: async (params: {
+    flow_type: 'troubleshooting' | 'procedural'
+    name: string
+    description: string
+    environment_tags?: string[]
+    category_id?: string
+  }): Promise<AIStartResponse> => {
+    const { data } = await apiClient.post('/ai/start', params)
+    return data
+  },
+
+  scaffold: async (conversationId: string): Promise<AIScaffoldResponse> => {
+    const { data } = await apiClient.post('/ai/scaffold', {
+      conversation_id: conversationId,
+    })
+    return data
+  },
+
+  branchDetail: async (
+    conversationId: string,
+    branchName: string
+  ): Promise<AIBranchDetailResponse> => {
+    const { data } = await apiClient.post('/ai/branch-detail', {
+      conversation_id: conversationId,
+      branch_name: branchName,
+    })
+    return data
+  },
+
+  assemble: async (
+    conversationId: string,
+    selectedBranches: Array<{
+      name: string
+      description: string
+      steps?: Record<string, unknown>
+    }>
+  ): Promise<AIAssembleResponse> => {
+    const { data } = await apiClient.post('/ai/assemble', {
+      conversation_id: conversationId,
+      selected_branches: selectedBranches,
+    })
+    return data
+  },
+}
+
+export default aiBuilderApi

frontend/src/api/index.ts

@@ -16,3 +16,4 @@ export { default as analyticsApi } from './analytics'
 export { targetListsApi } from './targetLists'
 export { maintenanceSchedulesApi, batchLaunchApi } from './maintenanceSchedules'
 export { default as feedbackApi } from './feedback'
+export { default as aiBuilderApi } from './aiBuilder'