fix: correct BC Admin Center API endpoints for database export

- Fix endpoint paths from /applications/businesscentral/environments/{env}/databaseExports
  to /exports/applications/BusinessCentral/environments/{env}
- Add Azure Storage SAS URI support (required by current export API)
- Update default API version from v2.15 to v2.21
- Add export metrics check before initiating export
- Use export history endpoint for status polling
- Download BACPAC from Azure Storage instead of expecting blobUri response

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bc-backup.conf.template

@@ -40,8 +40,28 @@ AZURE_CLIENT_SECRET=""
 # Find this in BC Admin Center: https://businesscentral.dynamics.com/
 BC_ENVIRONMENT_NAME=""
 
-# BC Admin API version (default: v2.15, adjust if needed)
-BC_API_VERSION="v2.15"
+# BC Admin API version (default: v2.21)
+BC_API_VERSION="v2.21"
+
+# ===================================
+# Azure Storage Configuration
+# ===================================
+# The BC Admin Center API exports the database to your Azure Storage account.
+# You need an Azure Storage account with a SAS URI that has Read, Write, Create, Delete permissions.
+#
+# To create a SAS URI:
+# 1. Go to Azure Portal > Storage Accounts > your account
+# 2. Go to "Shared access signature"
+# 3. Enable: Blob service, Container+Object resource types, Read+Write+Create+Delete permissions
+# 4. Set an appropriate expiry date
+# 5. Copy the generated SAS URL
+
+# Azure Storage Account SAS URI (full URI with SAS token)
+# Example: https://youraccount.blob.core.windows.net?sv=2021-06-08&ss=b&srt=sco&sp=rwdlac&se=...&sig=...
+AZURE_STORAGE_SAS_URI=""
+
+# Azure Storage container name for exports (will be created automatically)
+AZURE_STORAGE_CONTAINER="bc-exports"
 
 # ===================================
 # Encryption Configuration