Fix file size limit inconsistency: frontend allows 50MB but backend enforces 25MB

[emares17]

Summary

The frontend client-side validation allows files up to 50MB, but the server enforces a hard 25MB ceiling. Users with files between 25–50MB get no warning on the frontend but receive a server error on submit.

Root Cause

Three misaligned values:

• app.py:12 — MAX_CONTENT_LENGTH = 25 * 1024 * 1024 (Flask hard limit, request is rejected before reaching any route handler)
• backend/routes/uploads.py — /upload and /preview both hardcode if file_size_mb > 25:
• frontend-vite/react-ts/src/components/labelUploader.tsx — both handleUpload and handlePreview use const maxSize = 50 * 1024 * 1024

The settings.py config has MAX_FILE_SIZE = 50MB (dev) / 25MB (prod), but the route handlers ignore this config and hardcode 25MB directly.

Impact

Users who select a file between 25–50MB will pass client-side validation but get a 413 error from the server with no clear explanation.

Fix Options

1. Align frontend down to 25MB — matches the current server reality, no backend changes needed
2. Raise the limit to 50MB everywhere — update MAX_CONTENT_LENGTH in app.py, both route handlers in uploads.py, and ProductionConfig.MAX_FILE_SIZE in settings.py

Option 2 requires verifying the hosting tier supports 50MB request bodies before proceeding.

Notes

• This inconsistency pre-dates the label preview feature (PR Add label preview before full generation #9) — /upload had the hardcoded 25MB check before preview was added
• The route-level file_size_mb > 25 checks are redundant given MAX_CONTENT_LENGTH, but serve as a friendlier error message