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/schemas/__init__.py

@@ -5,6 +5,11 @@ from .session import SessionCreate, SessionUpdate, SessionResponse, SessionExpor
 from .category import CategoryCreate, CategoryUpdate, CategoryResponse, CategoryListResponse
 from .tag import TagCreate, TagResponse, TagListResponse, TagAssignment
 from .folder import FolderCreate, FolderUpdate, FolderResponse, FolderListResponse, FolderReorderRequest, FolderTreeRequest
+from .ai_builder import (
+    AIStartRequest, AIScaffoldRequest, AIBranchDetailRequest, AIAssembleRequest,
+    AIStartResponse, AIScaffoldResponse, AIBranchDetailResponse, AIAssembleResponse,
+    AIQuotaStatusResponse,
+)
 
 __all__ = [
     # User
@@ -21,4 +26,8 @@ __all__ = [
     "TagCreate", "TagResponse", "TagListResponse", "TagAssignment",
     # Folder
     "FolderCreate", "FolderUpdate", "FolderResponse", "FolderListResponse", "FolderReorderRequest", "FolderTreeRequest",
+    # AI Builder
+    "AIStartRequest", "AIScaffoldRequest", "AIBranchDetailRequest", "AIAssembleRequest",
+    "AIStartResponse", "AIScaffoldResponse", "AIBranchDetailResponse", "AIAssembleResponse",
+    "AIQuotaStatusResponse",
 ]

backend/app/schemas/ai_builder.py

@@ -0,0 +1,116 @@
+"""Pydantic schemas for the AI Flow Builder wizard."""
+from typing import Any, Literal, Optional
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+# ── Requests ──
+
+
+class AIStartRequest(BaseModel):
+    """Stage 1: Foundation — engineer provides flow metadata."""
+
+    flow_type: Literal["troubleshooting", "procedural"] = Field(
+        ..., description="Type of flow to generate"
+    )
+    category_id: Optional[UUID] = None
+    name: str = Field(..., min_length=1, max_length=255)
+    description: str = Field("", max_length=2000)
+    environment_tags: list[str] = Field(default_factory=list)
+
+
+class AIScaffoldRequest(BaseModel):
+    """Stage 2: Request AI-generated branch suggestions."""
+
+    conversation_id: UUID
+
+
+class AIBranchDetailRequest(BaseModel):
+    """Stage 3: Request AI-generated detail for one branch."""
+
+    conversation_id: UUID
+    branch_name: str = Field(..., min_length=1, max_length=255)
+
+
+class AIBranchUpdate(BaseModel):
+    """A branch with optional user edits for assembly."""
+
+    name: str
+    description: str = ""
+    steps: Optional[dict[str, Any]] = None
+
+
+class AIAssembleRequest(BaseModel):
+    """Stage 4: Assemble selected branches into a complete tree."""
+
+    conversation_id: UUID
+    selected_branches: list[AIBranchUpdate] = Field(..., min_length=2)
+
+
+# ── Responses ──
+
+
+class AIStartResponse(BaseModel):
+    """Response after creating a conversation."""
+
+    conversation_id: UUID
+    status: str
+
+
+class AIBranchSuggestion(BaseModel):
+    """A single branch suggestion from the AI."""
+
+    name: str
+    description: str
+
+
+class AIScaffoldResponse(BaseModel):
+    """Response with AI-suggested branches."""
+
+    conversation_id: UUID
+    branches: list[AIBranchSuggestion]
+    status: str
+
+
+class AIBranchDetailResponse(BaseModel):
+    """Response with AI-generated detail for one branch."""
+
+    conversation_id: UUID
+    branch_name: str
+    steps: dict[str, Any]
+    status: str
+
+
+class AITreeSummary(BaseModel):
+    """Summary statistics for an assembled tree."""
+
+    node_count: int
+    decision_count: int
+    action_count: int
+    solution_count: int
+    depth: int
+
+
+class AIAssembleResponse(BaseModel):
+    """Response with the fully assembled tree."""
+
+    tree_structure: dict[str, Any]
+    suggested_name: str
+    suggested_description: str
+    summary: AITreeSummary
+    status: str
+
+
+class AIQuotaStatusResponse(BaseModel):
+    """Current user's AI quota status."""
+
+    plan: str
+    monthly_used: int
+    monthly_limit: Optional[int]
+    monthly_reset_at: str
+    daily_used: int
+    daily_limit: Optional[int]
+    daily_reset_at: str
+    allowed: bool
+    ai_enabled: bool