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/main.py

@@ -13,7 +13,7 @@ from app.core.logging_config import setup_logging
 from app.core.middleware import RequestLoggingMiddleware, ErrorLoggingMiddleware
 from app.core.rate_limit import limiter
 from app.api.router import api_router
-from app.core.scheduler import scheduler, load_all_schedules
+from app.core.scheduler import scheduler, load_all_schedules, _cleanup_expired_ai_conversations
 
 # Initialize logging configuration
 setup_logging()
@@ -103,10 +103,17 @@ async def lifespan(app: FastAPI):
     # Note: In production, use Alembic migrations instead of init_db
     # await init_db()
 
-    # Start maintenance schedule runner
+    # Start maintenance schedule runner + AI conversation cleanup
     scheduler.start()
     async with async_session_maker() as db:
         await load_all_schedules(db)
+    scheduler.add_job(
+        _cleanup_expired_ai_conversations,
+        trigger="interval",
+        hours=1,
+        id="cleanup_ai_conversations",
+        replace_existing=True,
+    )
 
     # Auto-seed trees in background on PR environments
     seed_task = None