feat: add Script Builder service and API endpoints

- Script Builder service with language-specific system prompts (PowerShell, Bash, Python)
- AI-powered script generation with code block extraction and filename detection
- Context window management (last 20 messages) and session message limits
- REST API: CRUD sessions, send messages, save to Script Library
- Rate limiting on message endpoint (10/min), max 5 concurrent sessions per user
- Registered script_build action in AI model tier routing (standard tier)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/core/config.py

@@ -104,6 +104,7 @@ class Settings(BaseSettings):
         "open_chat": "standard",
         "variable_inference": "fast",
         "kb_convert": "standard",
+        "script_build": "standard",
     }
 
     def get_model_for_action(self, action_type: str) -> str: