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>

backend/app/models/__init__.py

@@ -26,6 +26,8 @@ from .user_pinned_tree import UserPinnedTree
 from .target_list import TargetList
 from .maintenance_schedule import MaintenanceSchedule
 from .feedback import Feedback
+from .ai_conversation import AIConversation
+from .ai_usage import AIUsage
 
 __all__ = [
     "User",
@@ -63,4 +65,6 @@ __all__ = [
     "TargetList",
     "MaintenanceSchedule",
     "Feedback",
+    "AIConversation",
+    "AIUsage",
 ]