Feat: adjust max file size to have customizable limit

[Benebo7]

Summary

Makes the 500MB per-file upload cap configurable via MAX_FILE_SIZE_MB
instead of hardcoded, so self-hosted operators can raise it for their
own hardware. Enforced on the backend; the frontend reads the same
value via runtime config so its pre-upload check stays in sync.

Changes

• adjust env example to a correct formatation, preventing error from direct copy — fixes an invalid ETL_SERVICE placeholder in surfsense_backend/.env.example that broke a direct copy-paste setup.
• add max file size env providing customizable limitation — introduces MAX_FILE_SIZE_MB, wired through the backend cap and the frontend runtime config.

Testing

Verified with MAX_FILE_SIZE_MB unset (default 500MB) and raised — in
both cases the frontend and backend limits stayed in sync, rejecting
files above the configured cap and accepting files within it end-to-end.
Confirmed the frontend keeps rejecting oversized files even after
raising the limit, until a file at or under the new cap is provided.

Obs

No UI changes so it`s not out of my codebase knowledge...

High-level PR Summary

This PR makes the 500MB per-file upload limit configurable through a new environment variable MAX_FILE_SIZE_MB, allowing self-hosted operators to adjust the cap based on their hardware capabilities. The change propagates the configurable limit through both backend enforcement and frontend pre-upload checks to keep them synchronized. Additionally, it fixes an invalid ETL_SERVICE placeholder in the backend environment example file that previously caused setup errors when copied directly.

⏱️ Estimated Review Time: 5-15 minutes

💡 Review Order Suggestion

Order	File Path
1	docker/.env.example
2	surfsense_backend/.env.example
3	docker/docker-compose.yml
4	docker/docker-compose.dev.yml
5	surfsense_backend/app/routes/documents_routes.py
6	surfsense_web/components/providers/runtime-config.server.tsx
7	surfsense_web/components/providers/runtime-config.tsx
8	surfsense_web/components/sources/DocumentUploadTab.tsx

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[Benebo7]

I am actually curious about storage limit per workspace specifically on official cloud, but I am not certain about whether it is a architectural decision or a missing config. Whatever, I would be happy to implement it if wanted...