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/config.py

@@ -72,6 +72,18 @@ class Settings(BaseSettings):
         """Check if Stripe is configured."""
         return self.STRIPE_SECRET_KEY is not None and self.STRIPE_WEBHOOK_SECRET is not None
 
+    # AI Flow Builder
+    ANTHROPIC_API_KEY: Optional[str] = None
+    AI_MODEL: str = "claude-haiku-4-5"
+    AI_CONVERSATION_TTL_HOURS: int = 24
+    AI_MAX_CALLS_PER_FLOW: int = 10
+    AI_REQUEST_TIMEOUT_SECONDS: int = 45
+
+    @property
+    def ai_enabled(self) -> bool:
+        """Check if AI Flow Builder is configured."""
+        return self.ANTHROPIC_API_KEY is not None
+
     # Deployment – auto-seed test data on PR environments
     SEED_ON_DEPLOY: bool = False