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/core/scheduler.py

@@ -1,4 +1,4 @@
-"""APScheduler integration for maintenance flow auto-session creation."""
+"""APScheduler integration for maintenance flow auto-session creation and AI cleanup."""
 import logging
 import uuid
 from datetime import datetime, timezone
@@ -7,8 +7,9 @@ from apscheduler.schedulers.asyncio import AsyncIOScheduler
 from apscheduler.schedulers.base import SchedulerNotRunningError
 from apscheduler.jobstores.base import JobLookupError
 from apscheduler.triggers.cron import CronTrigger
+from apscheduler.triggers.interval import IntervalTrigger
 import pytz
-from sqlalchemy import select
+from sqlalchemy import select, delete
 from sqlalchemy.ext.asyncio import AsyncSession
 
 logger = logging.getLogger(__name__)
@@ -114,6 +115,27 @@ async def _fire_maintenance_schedule(schedule_id: str) -> None:
             await db.rollback()
 
 
+async def _cleanup_expired_ai_conversations() -> None:
+    """Delete expired AI wizard conversations."""
+    import app.models  # noqa: F401
+    from app.core.database import async_session_maker
+    from app.models.ai_conversation import AIConversation
+
+    async with async_session_maker() as db:
+        try:
+            result = await db.execute(
+                delete(AIConversation).where(
+                    AIConversation.expires_at < datetime.now(timezone.utc)
+                )
+            )
+            if result.rowcount > 0:
+                logger.info(f"Cleaned up {result.rowcount} expired AI conversation(s)")
+            await db.commit()
+        except Exception:
+            logger.exception("Error cleaning up expired AI conversations")
+            await db.rollback()
+
+
 async def load_all_schedules(db: AsyncSession) -> None:
     """Load all active schedules into APScheduler on startup."""
     # Import all models to ensure SQLAlchemy mapper relationships resolve