docs: add MPS-6 through MPS-9 tickets, architecture diagram, update JS docs

- MPS-6: referrer analytics (domain, query, trend line charts)
- MPS-7: sandbox mode creative filter system
- MPS-8: user S3 bucket + artifact storage
- MPS-9: shop S3 mirror bucket
- architecture.md: system diagram, request flow, data pipeline, S3 layout
- JAVASCRIPT.md: add sandbox.js, signals.js, MediaPipe SDK entries
- sandbox-mode.md: mark S3 upload as implemented
- mps-2.md: document referrer_domain + referrer_query columns

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

docs/sandbox-mode.md

@@ -93,7 +93,7 @@ img.product-main
        │
        ├──> Download (a.download = "sandbox-export.png")
        │
-       └──> Upload (presigned POST to S3)  [future]
+       └──> Upload (presigned POST to user's S3 bucket)
 ```
 
 ### Video Export