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/ai_conversation.py

@@ -0,0 +1,67 @@
+"""AI Flow Builder conversation tracking.
+
+Stores wizard session state across the 4-stage flow builder process.
+Conversations expire after 24 hours and are cleaned up by the scheduler.
+"""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, Any
+
+from sqlalchemy import String, DateTime, ForeignKey, Integer, Text
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+from app.core.database import Base
+
+
+class AIConversation(Base):
+    __tablename__ = "ai_conversations"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    status: Mapped[str] = mapped_column(
+        String(20),
+        nullable=False,
+        default="foundation",
+        comment="foundation | scaffolding | detailing | reviewing | completed | expired",
+    )
+    # Conversation history across all wizard stages
+    messages: Mapped[list[dict[str, Any]]] = mapped_column(
+        JSONB, nullable=False, default=list
+    )
+    # Wizard state: Stage 1 metadata, Stage 2 branches, Stage 3 detail
+    wizard_state: Mapped[dict[str, Any]] = mapped_column(
+        JSONB, nullable=False, default=dict
+    )
+    # Assembled tree from Stage 4 (null until assembly)
+    generated_tree: Mapped[Optional[dict[str, Any]]] = mapped_column(
+        JSONB, nullable=True
+    )
+    # Tracks AI call count for per-flow limits
+    question_rounds: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0
+    )
+    expires_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+    )