Require X-Sync-Trigger-Key header on /api/sync endpoint

[Copilot]

• Add SyncTriggerKey config key to SyncController.cs (read from configuration)
• Validate X-Sync-Trigger-Key header in the /api/sync endpoint – return 401 if missing or incorrect, 500 if key is not configured
• Use string.Equals with StringComparison.Ordinal for key comparison
• Log error when SyncTriggerKey is not configured; print startup warning via Program.cs
• Update README.md Secrets reference section to document SyncTriggerKey

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a synchronization trigger key into configuration and require it when the synchronization is run</issue_title>
<issue_description>When running the synchronization endpoint /api/sync, the endpoint can be triggered anonymously. Add a key which will be required to trigger this specific endpoint, so that it cannot be triggered anonymously. The key should be configurable via configuration (also update Secrets reference in README). The key should be provided as a header X-Sync-Trigger-Key.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add a synchronization trigger key into configuration and require it when the synchronization is run #1

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[hajekj]

@copilot use equals instead of this comparison

[Copilot]

Done in b065b8c — replaced the CryptographicOperations.FixedTimeEquals byte-array comparison with string.Equals(providedKey.FirstOrDefault(), _syncTriggerKey, StringComparison.Ordinal) and removed the now-unused System.Security.Cryptography import.