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/api/router.py

@@ -28,6 +28,7 @@ from app.api.endpoints import notifications
 from app.api.endpoints import public_templates
 from app.api.endpoints import admin_gallery
 from app.api.endpoints import uploads
+from app.api.endpoints import script_builder
 
 api_router = APIRouter()
 
@@ -81,3 +82,4 @@ api_router.include_router(notifications.router)
 api_router.include_router(public_templates.router)
 api_router.include_router(admin_gallery.router)
 api_router.include_router(uploads.router)
+api_router.include_router(script_builder.router)