feat: add AI chat builder endpoints and update quota service

6 endpoints: create session, send message, get session, generate tree,
import to editor, abandon. Quota service daily limit updated to include
chat builder generation types.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/ai_quota_service.py

@@ -115,7 +115,7 @@ async def check_ai_quota(
         select(func.count(AIUsage.id)).where(
             AIUsage.user_id == user_id,
             AIUsage.succeeded == True,  # noqa: E712
-            AIUsage.generation_type.in_(["scaffold", "branch_detail"]),
+            AIUsage.generation_type.in_(["scaffold", "branch_detail", "chat_message", "chat_generate"]),
             AIUsage.created_at >= day_start,
         )
     ) or 0