fix: wrap sync task in Flask application context

[dahagag]

Problem

The celery sync task was failing silently with a RuntimeError when trying to commit database changes:

RuntimeError: No application found. Either work inside a view function or push an application context.

This happens because the async task executes outside of Flask's application context. When _mark_as_synced() tries to commit via db.session.commit(), Flask-SQLAlchemy needs an active application context to function.

Root Cause

Celery worker processes run independently from the Flask application. Database operations that depend on Flask-SQLAlchemy's request/app context must be explicitly wrapped.

Solution

Wrap the sync task execution in a Flask application context using with webapp.app_context():. This ensures SQLAlchemy session handling works correctly during database operations.

Changes

• Updated sync() task in tasks.py to wrap all operations in webapp.app_context()
• Maintains all existing logic and error handling
• No changes to dependencies or API

Testing

• ✅ Upload completes successfully (HTTP 200 to server)
• ✅ Database commit succeeds without RuntimeError
• ✅ Emails marked as synced in local database

Related

Complements systemctl service restart fix in PR #614