feat: Add Advoware History and Watcher services for document synchronization

- Implement AdvowareHistoryService for fetching and creating history entries.
- Implement AdvowareWatcherService for file operations including listing, downloading, and uploading with Blake3 hash verification.
- Introduce Blake3 utility functions for hash computation and verification.
- Create document sync cron step to poll Redis for pending Aktennummern and emit sync events.
- Develop document sync event handler to manage 3-way merge synchronization for Akten, including metadata updates and error handling.

services/exceptions.py

@@ -77,6 +77,11 @@ class EspoCRMTimeoutError(EspoCRMAPIError):
     pass
 
 
+class ExternalAPIError(APIError):
+    """Generic external API error (Watcher, etc.)"""
+    pass
+
+
 # ========== Sync Errors ==========
 
 class SyncError(IntegrationError):