Add remote server sync (data synchronization with upstream Sparked instance)

[OlivierJM]

Feature Parity: Remote Server Sync

Legacy app has a sync feature allowing one Sparked instance to pull content from another (upstream/remote) Sparked server. This is critical for offline/low-connectivity deployments where a local instance syncs from a central content server.

Legacy capabilities

• Admin authenticates with a remote Sparked server (via API credentials in config)
• Fetches collections from remote: courses, units, topics, resources, search data
• Database restoration from remote dumps
• Sync progress tracking UI with per-collection status
• Configuration via config.json (serverUrl, credentials)
• Supports multiple remote endpoints
• File synchronization (media files pulled from remote)

Use case

Field deployments of Sparked in low-bandwidth areas use a local server that periodically syncs from a central content server. Without sync, local instances must be manually updated.

Acceptance criteria

• Config: serverUrl and auth credentials for remote instance
• POST /api/sync/authenticate — verify remote server credentials
• POST /api/sync/fetch — pull specified collections from remote
• Progress tracking: per-collection sync status (in-progress, complete, failed)
• UI: sync management page (admin only) with trigger buttons and progress display
• Handle partial syncs gracefully (don't overwrite local data on failure)
• File/media sync from remote S3 or file server
• Sync history log

Identified via feature-parity audit against legacy Sparked (Meteor) app.